CCD-based images ( $1024\times 1024$ pixels) in the Johnson BVRI filters were obtained for PG0856+121 using the Thomson camera mounted on the Cassegrain focus of the 0.8-m IAC80 telescope (Teide Observatory) on April 13, 1998. The pixel size of the detector is 0.4325 $^{\prime\prime}$ . The night was photometric with an average seeing value around 2 $^{\prime\prime}$ . Raw frames were processed using standard techniques within the IRAF (Image Reduction and Analysis Facility) environment, which included bias subtraction, flat-fielding and correction for bad pixels by interpolation with values from the nearest-neighbour pixels. Landolt (1992) standard stars were also observed at various air masses in order to convert instrumental magnitudes into absolute data. Average rms values for the photometric calibration of each filter are as follows: 0.022 mag for B, 0.020 mag for V and R, and 0.018 mag for I. Photometry for the target star PG0856+121 was achieved via the point spread function (PSF) fitting method. The stellar PSF was determined for each colour frame using at least three "isolated'', bright point-like sources (not our target) appearing in the IAC80 images, and it was later applied to PG0856+121 providing the following colours and magnitudes in the Cousins photometric system: V=13.559, B-V=-0.311, R=13.667, R-I=-0.138.

Looking at the deconvolved R and I images of PG0856+121 it became evident that two faint red objects are present very close to the target; one object, named "B'' ( $I\,\le\,17.1$ , $R\,\le\,17.4$ ), located at $\le$ 2.4 $^{\prime\prime}$ eastward of our star, and another one, named "C'' ( $I\,=\,17.2\,\pm\,0.05$ , $R-I\,=\,1.2$ -1.6), located at about 3.5 $^{\prime\prime}$ northwest ( ${\rm PA}=296^{\circ}$ ) of it. If object "C'' is a Main Sequence star of solar metallicity, a spectral type of M2-M4 would be inferred for it. Figure 1 shows the R and I images of PG0856+121 in which the locations of the "B'' and "C'' point-like sources are indicated. Both sources are quite well detected when subtracting the average PSF from the central target. The typical PSF has FWHM values of 2.2 $^{\prime\prime}$ and 1.9 $^{\prime\prime}$ in the R and I frames, respectively. In view of this discovery, we now interpret the suggestion of UT98 for a binary nature of PG0856+121 in a different way: as they employed a 15 $^{\prime\prime}$ aperture for their JHK observations of the target, it is now clear that the two nearby red objects contributed significantly to their measurements. In particular, based on UT98 JHK values for PG0856+121 (see Table 1) and under the assumption of "C'' being a typical M-type field star, a contribution of 15% in the J band and of about 40% in the K band can be estimated. To test whether either of the two nearby objects is gravitationally linked to PG0856+121 by taking long-term radial velocity data sets is beyond the scope of the present work. In any case, we might be dealing with a detached system whose long period and orbital parameters should be tested against close binary evolution theories proposed for the hot subdwarfs (see, e.g., Iben & Tutukov 1986a,b; Iben 1990).

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{pg08_IR.2.eps}\end{figure}$

Figure 2: J-band image of PG0856+121 (central, bright star) confirming the presence of two nearby, faint sources labeled as "B" and "C" (see text)

On November 25, 2000, with the goal of proving objects "B'' and "C'', J images of PG0856+121 were obtained with the near-infrared camera (HgCdTe detector, $256\times 256$ ) mounted on the Cassegrain focus of the 1.5-m Carlos Sánchez Telescope (Teide Observatory). We performed the observations through the "narrow-optics'' of the instrument which provides a pixel projection of 0.4 $^{\prime\prime}$ onto the sky. The atmospheric seeing conditions during the night of the observations were fairly stable around 1.3 $^{\prime\prime}$ . The total integration time was 600 s, the final J image (Fig. 2) being the co-addition of five dithered exposures of 120 s each. Objects "B'' and "C'' around PG0856+121 are clearly resolved in this image, and thus proved to be real. The astrometric measurements carried out on this frame confirm those of the IAC80 observations for object "C'' and provide for object "B'' a separation of 2.3 $^{\prime\prime}$ at a position angle of 98 $^{\circ}$ from PG0856+121. Instrumental photometry has been performed using a similar procedure as described above and calibrated into real magnitudes with the observations of the standard star AS19-1 (Hunt et al. 1998), which were taken with the same instrumental configuration just after our target. The combined J magnitude of the three sources (PG0856+121 and objects "B'' and "C'') is $13.89\pm 0.10$ mag; this value is in agreement within the error bars of the near-infrared photometry given in UT98 for PG0856+121. Nevertheless, we have derived the J photometry for each star: 14.16 mag (78.3% is the contribution to the combined flux) for PG0856+121, 16.59 mag (8.3%) for object "B'', and 16.08 mag (13.3%) for object "C''. This latter object is the reddest of the three, and its contribution to the light in the near-infrared is indeed significant. Our J data confirms object "C'' as an early- to mid-M type dwarf, and would place it at a distance of $750\pm 350$ pc (adopting a main sequence calibration). This is only marginally consistent with the estimated distance of PG0856+121. The I-J colour of the hot subdwarf PG0856+121 is now -0.36 mag, which compares well to typical colours of other B type hot subdwarfs.

Unfiltered CCD photometry of PG0856+121 was also performed on February 27, 2000, using the same camera and telescope as for our previous observations (on April 13, 1998). The target was monitored every 37 s (25 s integration time plus overheads) during 2.4 hours in the airmass interval 1.04-1.20. The CCD detector was windowed so that two comparison stars of similar brightness in the field were observed simultaneously. We performed differential photometry of our target with an accuracy of the order of 0.005 mag. In brief, the procedure was as follows: apertures for PG0856+121 and the two reference stars were defined as a function of the average FWHM of the frame, and the sky intensity was set as an outer ring of width 1.5 pixel. We compared the reference stars against each other and found them to be constant at the level of our 1 $\sigma$ photometric error bars.

Spectral analysis of the February 27, 2000, data was achieved using the ISWF method over the frequency range 1 mHz to 15 mHz. The frequency spectrum is shown in Fig. 3 (amplitude square versus cyclic frequency). The horizontal line is the square of the mean value of the amplitude in the frequency range from 1 mHz to 15 mHz. The frequency resolution is 0.12 mHz. We found a significant peak at 3.30 mHz (303 s) with a signal-to-noise ratio of 3.6 in amplitude (roughly a 99% confidence level). This frequency peak does not differ within uncertainties from that derived by Piccioni et al. (2000), i.e. 3.2 mHz. This confirms the oscillatory nature of PG0856+121. Although with a smaller confidence level, peaks at 2.13 mHz (469.5 s, signal-to-noise of 2.3 in amplitude), and 1.99 mHz (502.5 s, signal-to-noise of 2.7 in amplitude) can be related to the one found by Piccioni et al. (2000) at 2.3 mHz with a similar signal-to-noise level. However, given the confidence level of our own measurements and those of Piccioni et al. (2000), we caution that the reliability of the shorter frequencies should be confirmed by further observations.

2 Observations and related results