2 Observations and data reduction

We have obtained high resolution spectra for all programme stars using the HIRES spectrograph at the Keck I telescope, the FEROS spectrograph at the ESO 1.5 m telescope, the FOCES spectrograph at the DSAZ 2.2 m telescope and the CASPEC spectrograph at the ESO 3.6 m telescope (see Table 1). Since the normalization procedure for the Echelle spectra is cumbersome for the rather broad Balmer lines (see below) it was deemed necessary to secure low resolution spectra at least for some of the programme stars to obtain independent estimates of the atmospheric parameters, in particular of the gravities. Appropriate spectra were obtained at Calar Alto and at ESO. Details are given in Table 1. For PG 1511+367, PG 1533+467 and PG 1610+239 we have new low resolution spectra presented in this paper. These spectra were taken with the TWIN spectrograph at the DSAZ 3.5 m telescope and reduced as described in Edelmann et al. (2001). The low resolution spectrum of PG 0122+214 were reduced as described in Moehler et al. (1997).

For the Keck-HIRES spectra observed in 1998 the blue cross disperser was used and the spectra cover the blue spectral range (3600 to 5130 Å). The standard data reduction as described by Zuckerman & Reid (1998) resulted in spectral orders that have a somewhat wavy continuum. Due to the merging of the higher Balmer lines the removal of the remaining waviness is difficult. We used the spectrum of H1504+65 (a very hot pre-white dwarf devoid of hydrogen and helium, Werner 1991), which was observed in the same night, for rectification of our spectra. Its spectrum has only few weak lines of highly ionized metals in the blue (3600-4480 Å) where the strong Balmer lines are found in the B-type stars. Therefore we normalized individual spectral orders 1 to 20 (3600-4480 Å) of the B-type stars by dividing through the smoothed spectrum of H1504+65. The remaining orders were normalized by fitting the continuum with spline functions (interpolated for orders 26 and 27 which contain H$\beta$). Judging from the match of line profiles in the overlapping parts of neighboring orders this procedure worked extremely well. In 1996 the red cross disperser of the Keck-HIRES spectrograph was used and therefore the blue part of the spectrum shortwards of 4200 Å was not recorded. Since no merging Balmer lines are present in this spectral range the waviness of the spectrum could be removed by fitting the continuum with spline functions. For orders containing broad Balmer lines the fit functions were interpolated between neighbouring orders.

The FOCES spectra were reduced as described in Pfeiffer et al. (1998) with a software package developed by the Munich Group. For the FEROS data the MIDAS reduction pipeline (François 1999) was used. For CASPEC data the procedure described by Heber et al. (1986) was applied. The CASPEC and FEROS spectra were normalized in a similar way as described for the Keck HIRES spectra of 1996.

Due to an error in the wavelength calibration no reliable radial velocity could be measured for HS 1914+7139.

 

 

Table 1: Observational parameters.

Name	Observation Date &	Telescope &	Resolution	Wavelength Range
	Time (UT)	Instrument	(FWHM Å)	(Å)
	Echelle spectra:
PG 0122+214	July 20, 1998 14:48	Keck HIRES	0.09	3600-5130
PG 1511+367	July 20, 1999 20:30	Calar Alto 2.2 m FOCES	0.15	3890-6995
PG 1533+467	July 19, 1999 21:00	Calar Alto 2.2 m FOCES	0.15	3890-6995
PG 1610+239	July 24, 1996 -	Keck HIRES	0.09	4265-6720
PG 2219+094	July 20, 1999 03:00	Calar Alto 2.2 m FOCES	0.15	3890-6995
PHL 159	Sep. 12, 1998 22:00	Calar Alto 2.2 m FOCES	0.15	3870-6830
PHL 346	Oct. 19, 1986 22:50	ESO 3.6 m CASPEC	0.20	4070-5130
SB 357	Oct. 1984	ESO 3.6 m CASPEC	0.20	4060-5090
HS 1914+7139	June 05, 1996 13:08	Keck HIRES	0.09	4280-6720
BD-15$^\circ$115	Sep. 08, 2000 07:49	ESO 1.5 m FEROS	0.09	3630-8860
	low resolution spectra:
PG 0122+214	Aug. 07, 1990 02:32	Calar Alto TWIN 3.5 m	3.5	3875-5010
PG 1511+367	July 18, 1999 20:49	Calar Alto TWIN 3.5 m	2.9	3350-7700
PG 1533+467	July 18, 1999 21:00	Calar Alto TWIN 3.5 m	2.9	3350-7700
PG 1610+239	Apr. 11, 2001	Calar Alto TWIN 3.5 m	1.0	4100-4950
PG 2219+094$^\star$	June 10, 1987 08:35	ESO MPIA 2.2 m B&C	2.5	4035-4900

$\parbox{15cm}{$^\star$\space Moehler et~al. (1990).}$