1 Introduction: HD 199143 and HD 358623

HD 199143 and HD 358623 were presented as young, nearby, co-moving stars by van den Ancker et al. (2000; henceforth vdA00). HD 199143 is at a distance of $47.7 \pm 2.4$ pc as measured by Hipparcos. Because HD 199143 and HD 358623 are only five arc min apart from each other and both show activity (a youth indicator) and the same proper motion, vdA00 suggested that they form a (small, but possibly larger) young nearby moving group (Capricornius), similar to the HorA, Tuc, TW Hya, and $\beta$ Pic moving groups. HD 199143 has similar UVW space motion as the Tuc and TW Hya stars (vdA00; Zuckerman & Webb 2000). Four more member candidates were presented by van den Ancker et al. (2001; henceforth vdA01) selected within $5^{\circ}$ around HD 199143 by strong ROSAT X-ray emission and, partly, by proper motion. It would be very important to find more members of this new association, not only to study the formation and mass function of these new young nearby groups, but also because such young nearby stars are very well suited for direct imaging searches of sub-stellar companions.

HD 199143 (also called BD $-17^{\circ}6127$ and SAO 163989) has spectral type F8 (Houk & Smith-Moore 1988; vdA00) and displays anomalous ultra-violett emission, Ca H & K emission, and fast rotation (vdA00). Also, HD 199143 shows N- and Q-band excess (vdA01) as well as IRAS $12~\mu{\rm m}$ excess emission, but very low upper limits in the other IRAS bands (vdA00), so that this star shows moderate, but not strong infrared (IR) excess emission. However, vdA00 argue that all its features could be explained by a faint T-Tauri-like companion, that itself produces the UV and IR excess emission and whose circumstellar material accretes onto HD 199143 and thereby spins up the primary. Mora et al. (2001) measured a rotational velocity of $v \cdot \sin i = 155 \pm 8$ km s^-1.

HD 358623 (also called BD $-17^{\circ}6128$ and AZ Cap) has spectral type K7-M0 with H$\alpha$ emission, strong flaring, and strong Li 6708 Å absorption (Mathioudakis et al. 1995; vdA00), as well as N- and Q-band excess (vdA01), all typical for a T Tauri star. The Tycho proper motion of HD 358623 ( $\mu _{\alpha} = 59$ $\pm$ 3 mas/yr and $\mu _{\delta} = -63$ $\pm$ 3 mas/yr) is identical to that of HD 199143 ( $\mu _{\alpha} = 59.2$ $\pm$ 1.1 mas/yr and $\mu _{\delta} = -61.55$ $\pm$ 0.85 mas/yr). Recently, Zuckerman et al. (2001) argued that both HD 199143 and HD 358623 are actually part of the $\beta$ Pic moving group.

In May 2001, Jayawardhana & Brandeker (2001; henceforth JB01) observed the two stars in J and K with the AO system ADONIS at the ESO 3.6 m telescope on La Silla. Near each of the two stars, they detected one companion candidate. The close and faint object near HD 199143 is red (J-K=1.4 mag) and, hence, either sub-stellar or the (reddened) companion expected by vdA00, whose circumstellar material could explain the anomalous features of the primary HD 199143. The companion candidate near HD 358623 is less than 2 mag fainter than the primary in J and K (JB01) and could be an M-type stellar companion.

We observed the two stars with high sensitivity and high dynamic range in order to detect faint companions, namely in the H-band in Dec. 2000, July 2001, and Dec. 2001 and in the J- and K-bands in July 2001 (speckle and normal IR imaging). Young nearby stars like HD 199143 and HD 358623 are well-suited for direct imaging of sub-stellar companions, both brown dwarfs and giant planets, because young sub-stellar objects are still relatively bright (e.g. Burrows et al. 1997), so that they are less difficult to detect in the PSF wing of a much brighter star. See, e.g., Lowrance et al. (1999, 2000), Neuhäuser et al. (2000b), and Guenther et al. (2001) for imaging and spectroscopy of brown dwarf companions of the young stars TWA-5 and HR 7329 in the TW Hya and Tuc associations. The brown dwarf near TWA-5 was the first sub-stellar companion around a pre-main sequence star confirmed by both proper motion and spectroscopy, and also the first around a spectroscopic binary (Torres et al. 2001), and the first around a star with evidence for a disk (Jayawardhana et al. 1999). While the first four brown dwarfs confirmed as companions all orbit M-type stars, an A-type star also can have a brown dwarf companion (e.g. HR 7329 with spectral type A0). Therefore, both HD 199143 (F8) and HD 358623 (K7-M0) are promising targets for the direct imaging search for sub-stellar companions.

The probability for the two companion candidates detected by JB01 to be unrelated background objects happening to lie in the line-of-sight next to the primary stars is very small (JB01). However, one should not rely on such probabilities, even when observing only a small sample. Some previous very faint sub-stellar companion candidates (e.g. Terebey et al. 1998; Neuhäuser et al. 2000a) with very low background probability were found to be background stars by follow-up spectroscopy (Terebey et al. 2000; Neuhäuser et al. 2001). This shows how important it is to take multi-epoch images and spectra.

We present our imaging observations in Sect. 2 and the resulting photometry for the two stars and their companion candidates in Sect. 3. Astrometry is presented in Sect. 4 to check whether the companion candidates are co-moving with their putative primary stars. Then, in Sect. 5, we present an H-band spectrum of one of the two companion candidates. We conclude in Sect. 6.