Hot B-type subdwarfs (sdBs) are H-rich blue subluminous objects with temperatures not exceeding about 35000 K (Greenstein & Sargent 1974; Heber 1986). They have a canonical mass of 0.55 $M_{\odot}$ , with thin H-rich envelopes of less than 0.02 $M_{\odot}$ , and a distribution in logg around 5.25-6.5 (Ulla & Thejll 1998, hereafter referred to as UT98). These objects are proposed to be progenitors of white dwarfs and descendants of blue horizontal branch stars or asymptotic giant branch (AGB) stars (Saffer et al. 1998). They are also proposed to be responsible for the UV upturn flux observed in early-type galaxies (Bica et al. 1996). Among the various theories for the origin and final fate of the sdBs, close binary evolution has been suggested as one of the likely channels. These investigations are relevant to the formation of type Ia supernovae by merging of double-degenerate pairs, in which one or both members could be descendants of hot subdwarfs (Saffer et al. 1998). Enough evidence has been accumulated to date in favor of a binary nature for at least 40% of the field hot B subdwarf stars (e.g. Allard et al. 1994; Jeffery & Pollacco 1998; UT98), with the detected companions ranging broadly in spectral type and physical parameters.

It is therefore very important to continue to seek information on the current binary nature of hot subdwarfs. In this regard and based on JHK photometry, Thejll et al. (1995, hereafter referred to as TUM95) and UT98 have compiled a list of suitable candidates. They also suggested some particular targets for further investigation, despite large error bars associated with the observations (see UT98 for details). A way to pursue a more detailed study of the binary nature of such objects is to obtain filtered CCD imaging to search for close red components whose IR emission could have contributed to TUM95 and UT98 measurements. With that aim we have started such a program and present here results for the sdB star PG0856+121. Table 1 summarizes relevant information about this object published to date.

Table 1: Summary of properties of PG 0856+121 (references provided)
Names: PG 0856+121 (1) WD 0856+121 (2)

Sp.type: sdB (1; 3) non variable (4)

RA(1950)= 08 56 18.8 Dec = +12 08 06 (1)

(2000) 08 59 02.723 +11 56 24.73 (5)

LII = 216.56 BII = 33.67 (3)

l = 216.49 b = 33.68 (5)

distance: (pc) 990 $\pm$ 370 (5, 6)

heigh: (pc) 550 $\pm$ 210 (3)

rad. vel.: (kms^-1) +85 (5, 6)

+97 $\pm$ 10.2 (4)

prp. mot.: $\mu_\alpha$ cos $\alpha$ $\mu_\delta$ (mas/yr)

-19.4 -19.8 (5, 6)

orbital components, velocity and parameters: (5)

X = -9.16 Y = -0.49 Z = 0.55 (kpc)

U = -74 V = 116 W = -46 (kms^-1)

It (kpc kms^-1)= -1099 ecc = 0.48 nze = 0.14

magnitudes:

B V R I

13.248 13.559 13.667 13.805

$\pm$ 0.022 $\pm$ 0.020 $\pm$ 0.020 $\pm$ 0.018 (7)

B $P_{\rm mag}$ U-B B-V

13.28 13.03 -1.03 -0.19 (1)

v u-v g-v g-r

13.52 -0.11 -0.15 -0.52 (1)

y b-y m1 c1

13.47 -0.094 +0.106 -0.004 (8)

13.50 -0.116 +0.113 +0.021 (8)

13.473 -0.095 -0.094 +0.035

$\pm$ 0.012 $\pm$ 0.004 $\pm$ 0.005 $\pm$ 0.007 (9)

13.495 -0.116 +0.113 u-b

$\pm$ 0.030 $\pm$ 0.032 $\pm$ 0.036 .015 $\pm$ .021 (10)

J H K E(B-V)

13.42 13.59 13.84 $\le$ 0.025

$\pm$ 0.44 $\pm$ 0.22 $\pm$ 0.55 (11)

f₆₇₀₀/f₆₀₅₀ = 0.651 f₇₀₅₀/f₆₇₀₀ = 0.840 (12)

$T_{\rm eff}$ : (K) 22000 23800 33000 (3)

26400 (13)

$\log(g)$ : 5.1 (9) 5.73 (Y=0.001) (13)

optical sp.: (9); finder chart: (1)

1: Green et al. (1986); 2: McCook & Sion (1987); 3: Moehler et al. (1990a);

4: Saffer et al. (1998); 5: de Boer at al. (1997); 6 : Colin et al. (1994);

7: this work; 8: Kilkenny et al. (1988); 9: Moehler et al. (1990b);

10: Wesemael et al. (1992); 11: UT98; 12: Jeffery & Pollacco (1998);

13: Saffer et al. (1994).

**Table 1:** Summary of properties of PG 0856+121 (references provided)
Names:	PG 0856+121	(1)	WD 0856+121	(2)
Sp.type:	sdB	(1; 3)	non variable	(4)
RA(1950)=	08 56 18.8	Dec =	+12 08 06	(1)
(2000)	08 59 02.723		+11 56 24.73	(5)
LII =	216.56	BII =	33.67	(3)
l =	216.49	b =	33.68	(5)
distance:	(pc)	990	$\pm$ 370	(5, 6)
heigh:	(pc)	550	$\pm$ 210	(3)
rad. vel.:	(kms^-1)	+85		(5, 6)
		+97	$\pm$ 10.2	(4)
prp. mot.:	$\mu_\alpha$ cos $\alpha$	$\mu_\delta$	(mas/yr)
	-19.4	-19.8		(5, 6)
orbital components, velocity and parameters: (5)
X = -9.16	Y = -0.49	Z = 0.55	(kpc)
U = -74	V = 116	W = -46	(kms^-1)
It (kpc kms^-1)=	-1099	ecc = 0.48	nze = 0.14
magnitudes:
B	V	R	I
13.248	13.559	13.667	13.805
$\pm$ 0.022	$\pm$ 0.020	$\pm$ 0.020	$\pm$ 0.018	(7)
B	$P_{\rm mag}$	U-B	B-V
13.28	13.03	-1.03	-0.19	(1)
v	u-v	g-v	g-r
13.52	-0.11	-0.15	-0.52	(1)
y	b-y	m1	c1
13.47	-0.094	+0.106	-0.004	(8)
13.50	-0.116	+0.113	+0.021	(8)
13.473	-0.095	-0.094	+0.035
$\pm$ 0.012	$\pm$ 0.004	$\pm$ 0.005	$\pm$ 0.007	(9)
13.495	-0.116	+0.113	u-b
$\pm$ 0.030	$\pm$ 0.032	$\pm$ 0.036	.015 $\pm$ .021	(10)
J	H	K	E(B-V)
13.42	13.59	13.84	$\le$ 0.025
$\pm$ 0.44	$\pm$ 0.22	$\pm$ 0.55		(11)
f₆₇₀₀/f₆₀₅₀ =	0.651	f₇₀₅₀/f₆₇₀₀ =	0.840	(12)
$T_{\rm eff}$ : (K)	22000	23800	33000	(3)
	26400			(13)
$\log(g)$ :	5.1	(9)	5.73 (Y=0.001)	(13)
optical sp.: (9); finder chart: (1)
1: Green et al. (1986); 2: McCook & Sion (1987); 3: Moehler et al. (1990a);
4: Saffer et al. (1998); 5: de Boer at al. (1997); 6 : Colin et al. (1994);
7: this work; 8: Kilkenny et al. (1988); 9: Moehler et al. (1990b);
10: Wesemael et al. (1992); 11: UT98; 12: Jeffery & Pollacco (1998);
13: Saffer et al. (1994).

PG0856+121 was suspected to be a possible pulsating sdB candidate by Koen et al. (1997, 1998a), based on the similarity of its physical properties to those of known sdB pulsating - or EC14026 - stars (Kilkenny et al. 1997). This suspicion was confirmed by Piccioni et al. (2000), who found periodic light variations with frequencies of 2.3 mHz and 3.2 mHz at a reasonable confidence level. We present our CCD and near-IR observations in Sect. 2. Section 3 provides a brief description of the models used and analysis performed to investigate the oscillatory nature of PG0856+121. Our conclusions are presented in Sect. 4.

1 Introduction