4 Summary and conclusions

Optical (BVRI) and near-IR (J) imaging of the field nearby the sdB star PG 0856+121 revealed the presence of two faint red objects very close ($\le$4 $^{\prime\prime}$) to the target. In view of this discovery, contamination by them in the near-IR bands is here proposed as the most likely interpretation for the previously reported JHK values for the object (UT98). Our photometric data show that the optical and near-IR colours of PG0856+121 are consistent with those of other single hot subdwarfs. Whether either of the two red objects is gravitationally linked to PG 0856+121 has not been investigated further but it is now brought to the attention of potentially interested researchers. If a binary nature could be established for the target, a refinement in the determination of its physical properties together with those of its companion would be obtainable, in the ways abundantly documented in the literature already. It is worth noting that if the given distance (Table 1) of 990 pc to the target is correct, then the reddest "C" companion, for which an early- to mid-M type dwarf has been determined given its IR colours, would be placed at a distance of $750\pm 350$, marginally consistent with the estimated distance of PG0856+121 above. This makes it very difficult to measure radial velocities by the usual spectroscopic techniques. On the other hand, the presence of an even closer (and therefore unresolved) companion to PG 0856+121 could still be revealed through radial velocity measurements. As a further suggestion, checking of the eventual binary nature of PG 0856+121 towards either the "C" or "B" objects, could also be possible on an approximate time-scale of 10 years by means of its proper motion values, as provided by de Boer et al. (1997) and Colin et al. (1994).

Recently, PG 0856+121 has been reported to display a pulsating nature by Piccioni et al. (2000). The new differential photometry of the target presented here mostly confirms the peaks, at 2.3 and 3.2 mHz, detected by these authors. In both works, the largest frequency peak is found at $3.3\pm 0.1$ mHz and, also in both works, peaks around 2.0-2.3 mHz are found, although with a lower confidence level. We have compared these frequency peaks with those of stellar models compatible with the physical properties of the target star. Our results indicate that the peak at 3.3 mHz is a p- or g-mode with a low radial order; in particular, if the p-mode character is assumed and the canonical mass is considered, it would be the fundamental mode with degree $\ell=0,1$ or 2.

However, the other frequency peaks are in the g-mode range of our models. Since other pulsating sdB stars seem to have only p-modes, in agreement with earlier theoretical computations, alternative explanations for our low frequency peaks need to be considered. First, the S/N ratio for these peaks is rather small and, hence, require further confirmation. Also we note that the results are based on the stellar parameters provided by Saffer et al. (1994) - mainly the value of $\log g$ -. To search for systematic shifts in these parameters would demand not only spectra of higher S/N ratio but also to test effects such as the importance of considering NLTE atmospheric models for the sdB stars. In view of the importance of these considerations for the particular case of PG 0856+121, we propose it as a candidate for future improved spectroscopic studies in an attempt to further constrain its oscillatory properties. On the other hand, we find it unlikely that this result arises from errors in the stellar models analysis performed; in particular, as indicated in Sect. 3, even if the models considered were unsuitable for the target, the p-mode range of frequencies has a small dependence on the models' details, and a broad mass range (up to $1~M_{\odot}$) has been tested still yielding low frequency peaks in the g-mode range.

Acknowledgements

The authors are grateful to V.J.S. Béjar for his assistance in obtaining the J images at the CST on November 25, 2000. Ana Ulla acknowledges support from the Spanish MEC DGESIC under contract PB97-1435-C02-02 and from the Spanish MCT DGI under contract AYA2000-1691. This research made use of the SIMBAD database, operated at CDS, Strasbourg, France and of the NASA Astrophysics Data System (ADS).