To obtain an upper limit to our resolution we tried to estimate the Rbrightness of the cool companion by fitting the available photometric data of those stars that have sufficient measurements. In order to disentangle the flux of the hot star from that of the cool star we analyse the composite spectral energy distribution. For this purpose ultraviolet, optical and infrared (spectro-) photometry is collected from literature and archives (IUE, 2MASS). To determine the contribution of the hot star we fit synthetic spectra (Kurucz 1992) to the bluest part of the observed spectral range, i.e. IUE data plus u or u/U plus v/B (if no UV data were available) and determine the effective temperature of the sdB star. In doing so we assume that the companion does not contribute to the flux in this wavelength range (cf. Fig. 5). While this is probably true for the IUE data, some contamination may be present in the u/U- and v/B-band and consequently the temperature determination for the sdB star can be compromised.

However, for some stars photometric data are so incomplete that no meaningful fit can be obtained. Aside from the F675W measurements discussed here PG 0942+461 and HE 2213-2212 have only JHK photometry from 2MASS, which are insufficient for a fit. While HE 0430-2457 has BVR photometry it is still not possible to constrain the sdB star's temperature with these data as B-V is insensitive to $T_{\rm eff}$ at sdB temperatures. To convert the magnitudes into flux values we used the data given in Table 3.

**Table 3:** Flux for a star with $m_\lambda$ = 0. The data are taken from Lamla (1982, p. 59, *uvby*; p. 82 $BVR_{\rm C}I_{\rm C}$ ), Zombeck (1990, $JHK_{\rm UT98}$ ) and from the 2MASS Team (priv. comm., $JHK_{\rm 2MASS}$ ).
filter	flux	$\lambda_{\rm c}$
	[erg/(cm² s Å)]	[Å]
u	$1.169\times 10^{-8}$	3500
v	$8.444\times 10^{-9}$	4110
b	$5.826\times 10^{-9}$	4670
y	$3.700\times 10^{-9}$	5470
U	$4.187\times 10^{-9}$	3600
B	$6.597\times 10^{-9}$	4400
V	$3.607\times 10^{-9}$	5500
$R_{\rm C}$	$2.254\times 10^{-9}$	6400
$I_{\rm C}$	$1.196\times 10^{-9}$	7900
$J_{\rm 2MASS}$	$2.91\times 10^{-10}$	12510
$H_{\rm 2MASS}$	$1.11\times 10^{-10}$	16280
$K_{\rm 2MASS}$	$3.83\times 10^{-11}$	22030
$J_{\rm UT98}$	$3.18\times 10^{-10}$	12500
$H_{\rm UT98}$	$1.18\times 10^{-11}$	16500
$K_{\rm UT98}$	$4.17\times 10^{-11}$	22000

By comparing the measured flux in the R band to the model flux of the sdB star we derive the flux ratio of the hot vs. the cool star in the system. For those systems which should have a rather bright companion according to their photometric data we verified the flux ratio in R between sdB and cool companion from two colour diagrams similar to those used by Ferguson et al. (1984), which is best suited for components of comparable brightness (for details see Ferguson et al. 1984). With this method we found that the companion of TON 1281 is bright enough to affect also the u filter, yielding a temperature of 25000K to 27000K for the sdB instead of the 22000K given in Table 4 and a brightness difference $\Delta R$ of $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ to $\rm -0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$ . Also for PG 1601+345 we find a much smaller brightness difference ( $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$ ) and higher temperature (29500 K) from this method than from our photometric fits. In this case the B filter is already affected by the cool companion. For reasons of consistency we keep the values from the photometric fits for these two stars in Table 4. For all other stars with brightness differences $\le$ $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ the results from both methods were the same. To correct for interstellar reddening we used the reddening-to-infinity maps of Schlegel et al. (1998) which give somewhat higher values than the older data of Burstein & Heiles (1982). KPD 2215+5037, PG 1558-007, and PG 2259+134 all lie in regions of quite high reddening according to Schlegel et al. (1998) and show no spectroscopic evidence for a cool companion (see Appendix A). The observed apparent infrared excess can be explained by high interstellar reddening alone, without invoking the presence of a cool companion. We also find no evidence for a companion from available photometry of PG 1656+213, although there is spectroscopic evidence (Ferguson et al. 1984). However there are are no flux measurements redwards of V available and B and V fluxes are inconsistent. Therefore we keep PG 1656+213 as a programme star.

Table 4: Estimated temperature of sdB stars, resulting reddening-free brightness of subdwarf B star ( $R_{\rm sdB,0}$ ) and companion ( $R_{\rm comp,0}$ ), distance d, brightness difference $\Delta R$ , and upper limit for linear separation a $_{\rm lim}$ derived from upper limit of angular separation $\Delta \alpha _{\rm lim}$ . The reddening estimates are from the maps of Schlegel et al. (1998) and we used $A_R = 2.6 \cdot$ E_B-V. The three different temperatures for PG 1511+624 result from the three available SWP spectra. If no evidence for a companion can be found from available photometry no entry is given in Col. 4.
Star $T_{\rm eff, sdB}$ A_R $R_{\rm comp,0}$ $R_{\rm sdB,0}$ $M_{R, {\rm sdB}}$ d $\Delta R$ $\Delta \alpha _{\rm lim}$ $a_{\rm lim}$

[K] [pc] [AU]

PB 6107 23000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}086$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 870 $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1 87

PG 0105+276 32000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}156$ $\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 1100 $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1 110

PHL 1079 25000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}104$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 630 $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1 63

PG 0749+658 22000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}125$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ $\rm 12\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 580 $\rm 2\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 2 116

TON 1281 22000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}065$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 1150 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 80

TON 139 20000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}026$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 950 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 05 48

PG 1309-078 24000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}138$ $\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 910 $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1 91

PG 1421+345 24000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}044$ $\rm 16\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 2100 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1 210

PG 1449+653 28000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}042$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 830 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 58

PG 1511+624 31000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}047$ $\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 1200 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 84

28000 $\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 1200 $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 84

33000 $\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 1260 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 88

PG 1558-007 23000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}468$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 910

PG 1601+145 25000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}133$ $\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 1100 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 77

PG 1636+104 20000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}156$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 1200 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 84

PG 1656+213 17000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}172$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$ $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ 1800

TON 264 26000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}146$ $\rm 16\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 870 $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 2 174

PG 1718+519 27000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}081$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 950 $\rm -0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 05 48

PG 2148+095 26000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}169$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$ $\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 520 $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1 52

KPD 2215+5037 35000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}871$ $\rm 12\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 480

PG 2259+134 30000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}341$ $\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 1000

BD $-7^\circ$ 5977 29000 $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}093$ $\rm 10\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ $\rm 11\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ 320 $\rm -1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$ 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 2 64

**Table 4:** Estimated temperature of sdB stars, resulting reddening-free brightness of subdwarf B star ( $R_{\rm sdB,0}$ ) and companion ( $R_{\rm comp,0}$ ), distance d, brightness difference $\Delta R$ , and upper limit for linear separation a $_{\rm lim}$ derived from upper limit of angular separation $\Delta \alpha _{\rm lim}$ . The reddening estimates are from the maps of Schlegel et al. (1998) and we used $A_R = 2.6 \cdot$ E_B-V. The three different temperatures for PG 1511+624 result from the three available SWP spectra. If no evidence for a companion can be found from available photometry no entry is given in Col. 4.
Star	$T_{\rm eff, sdB}$	A_R	$R_{\rm comp,0}$	$R_{\rm sdB,0}$	$M_{R, {\rm sdB}}$	d	$\Delta R$	$\Delta \alpha _{\rm lim}$	$a_{\rm lim}$
	[K]					[pc]			[AU]
PB 6107	23000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}086$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	870	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1	87
PG 0105+276	32000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}156$	$\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	1100	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1	110
PHL 1079	25000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}104$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$	$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	630	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1	63
PG 0749+658	22000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}125$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	$\rm 12\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	580	$\rm 2\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 2	116
TON 1281	22000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}065$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	1150	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07	80
TON 139	20000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}026$	$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$	$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	950	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 05	48
PG 1309-078	24000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}138$	$\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	910	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1	91
PG 1421+345	24000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}044$	$\rm 16\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	2100	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1	210
PG 1449+653	28000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}042$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	830	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07	58
PG 1511+624	31000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}047$	$\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	1200	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07	84
	28000		$\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	1200	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07	84
	33000		$\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	1260	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07	88
PG 1558-007	23000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}468$		$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	910
PG 1601+145	25000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}133$	$\rm 15\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	1100	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07	77
PG 1636+104	20000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}156$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$	$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	1200	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07	84
PG 1656+213	17000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}172$		$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	1800
TON 264	26000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}146$	$\rm 16\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	870	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 2	174
PG 1718+519	27000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}081$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	950	$\rm -0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 05	48
PG 2148+095	26000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}169$	$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$	$\rm 13\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	520	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1	52
KPD 2215+5037	35000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}871$		$\rm 12\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	480
PG 2259+134	30000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}341$		$\rm 14\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	1000
BD $-7^\circ$ 5977	29000	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}093$	$\rm 10\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$	$\rm 11\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$	$\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$	320	$\rm -1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 2	64

Aznar Cuadrado & Jeffery (2001) present an extensive discussion of sdB parameters derived from energy distributions, which also includes some of the stars discussed in this paper. In Table 5 we present the temperatures given in their paper and other values collected from literature in comparison to the ones derived here. As can be seen from Table 5 differences of $\pm$ 10% in $T_{\rm eff}$ between different authors are quite common.

**Table 5:** Effective temperatures for sdB stars derived from energy distributions by various authors. The sources are Aznar Cuadrado & Jeffery (2001, ACJ01), Allard et al. (1994, A94), Theissen et al. (1993, T93; 1995, T95), Ulla & Thejll (1998, UT98).
star	$T_{\rm eff}$ [K] derived by
	this paper	ACJ01	T93	A94	T95	UT98
PB 6107	23000			25000
PG 0105+276	32000	35850		32000
PHL 1079	25000		26350		30000	30000
PG 0749+658	22000	25050		23500
TON 1281	22000	23275		29500
TON 139	20000					18000
PG 1449+653	28000	28150		28000
PG 1511+624	31000:			33000
PG 1636+104	20000			21000
TON 264	26000			28500
PG 1718+519	27000	29950	23500	25000	30000
PG 2148+095	26000	22950		26000		25000
KPD 2215+5037	35000			24500
PG 2259+134	30000	28300	28500		22500

The temperatures derived from the photometric data and from line profile fits for the stars in regions with high reddening agree moderately well (compare Tables 4 and A.1). The discrepancies may be due to small scale variations in reddening that affect the temperatures derived from photometry but not those derived from line profile fits.

From the photometric fit we can derive the apparent R magnitudes of the sdB and of the cool star and correct both for interstellar extinction. The uncertainty in $T_{\rm eff}$ of about $\pm$ 10% evident from Table 5 causes an estimated uncertainty in the derived brightness for both components of $\rm\pm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ . Knowing the absolute Rmagnitude of the sdB stars then allows to determine their distance. We use the mean M_V derived by Moehler et al. (1997) for hot subdwarfs in the globular cluster NGC 6752. They found two groups of hot subdwarfs, a cooler one with a mean effective temperature of 22000K and <M_V> = $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ (5 stars), and a hotter one with < $T_{\rm eff}$ > = 29000K and <M_V> = $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}2$ (12 stars). From Kurucz (1992) model atmospheres for [M/H] = 0 we find V-R = $\rm -0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}120$ for $T_{\rm eff}$ = 22000K and $\rm -0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}152$ for 29000K. We therefore use M_R = $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}3$ for stars cooler than 25000K and M_R = $\rm 4\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ for hotter stars.

Using the archive point spread functions we estimated the minimum separation that we can resolve for a given brightness difference by adding two PSFs with a defined brightness difference and angular separation and examining the resulting image by eye. We find the following resolution limits: $\Delta \alpha _{\rm lim}$ ( $\Delta R$ ) = 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 2 ( $\rm 2\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ ), 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 1 ( $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$ ), 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 07 ( $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}0$ ), 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 05 ( $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}5$ ). Using the distances determined above we can now derive upper limits for the linear separation of the unresolved binaries (cf. Table 4), ranging from 50 AU to 210 AU.

Table 2 shows that the brightness differences between the components in TON 1281 and HE 0430-2457 are too large to reproduce the spectral energy distribution of TON 1281 and the photometry of HE 0430-2457, respectively. The large brightness difference of $\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$ (from the WFPC2 data) for PG 1558-007 agrees with the lack of photometric and spectroscopic evidence for a companion. In the remaining two cases (PG 1718+519, TON 139) the brightness differences in Table 2 are somewhat larger than those derived from the spectral energy distribution. To see whether we can in principle accommodate the HST observations by fits to the photometric data we repeated the fits, this time enforcing the brightness difference in the R band obtained from the HST data. The results are shown in Fig. 5 (in comparison to the original fits). Obviously the companion of PG 1718+519 is sufficiently bright to affect also the u filter, thereby rendering our assumption that this filter is unaffected by the cool companion obsolete. The fits for TON 139 do not show much difference. We conclude that the spectral energy distribution of TON 139 and PG 1718+519 are consistent with the R band flux ratio measured with the HST WFPC2 camera.

3.1 The sdO star PG 0105+276

Since the He-sdO PG 0105+276 does not belong to the programme sample, we discuss it separately. It is the only programme star that is resolved into three components. However, the two companions are quite distant from the primary (3 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 37 and 4 $.\!\!{\hbox{$^{\prime\prime}$ }}$ 48, respectively). The light of these companions can explain at least qualitatively the IR excess observed by ground based aperture photometry. The spectrum of PG 0105+276, however, does not show any signature of a cool companion, probably because due to the orientation and the small width of the slit no light of the distant companions was included. The diaphragm used in the photometry was large (18 $^{\prime \prime }$ ) and included the companions' light.

The brightness differences measured on the WFPC2 image ( $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ , $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$ ) for PG 0105+276 are smaller than the one derived from the photometric fit ( $\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}4$ ), i.e. one companion is brighter than expected. However, as discussed in Appendix A, the true temperature (from line profile fitting) is much higher than the one obtained from the spectral energy distribution (63000K vs. 35000K) making the companion's luminosity obtained from photometry a lower limit only.

3 Spectral energy distribution

3.1 The sdO star PG 0105+276