1 Introduction

Large samples of stellar X-ray sources have been discovered serendipitously by the Einstein, EXOSAT and ROSAT Observatories. Optical follow-up studies have demonstrated that a large fraction of these samples is composed by active binaries, by pre-main sequence and other very young stars and by flare stars (see, among others, Fleming et al. 1988, 1989a, 1989b; Favata et al. 1993, 1995; Jeffries et al. 1995; Neuhäuser et al. 1997).

Over the past few years our group has carried out extensive photometric and spectroscopic observations of cool stars serendipitously detected by EXOSAT (Tagliaferri et al. 1992, 1994; Cutispoto et al. 1996). We have shown that, in addition to dMe flare stars, at least one third of the EXOSAT serendipitous sources is constituted by young stars, with ages comparable to or younger than the Pleiades. Another third consists of RS CVn binaries, while the physical nature of the remaining sources is more uncertain: they could be either young objects or very active binaries. A large fraction of these stars is variable at optical wavelengths, with the observed variability best interpreted as produced by photospheric cool spots. Similar results have been obtained for the samples of cool stars selected with the Einstein satellite (Fleming et al. 1988, 1989a, 1989b; Favata et al. 1993, 1995).

Probably, the most interesting result of these surveys is that there seems to be an excess of young stars, near ZAMS or even younger, with respect to what is predicted by Galaxy models. This result is now confirmed by other authors using ROSAT data. For instance, Jeffries (1995) determined the Li abundances for a sample of late-type stars EUV-selected with the WFC on board ROSAT. He confirmed that a high portion of these stars are as young as or younger than the Pleiades and, based on their kinematics, he suggested that these stars are part of a group of young open clusters and nearby B stars known as the Local Association. Finally, Neuhäuser et al. (1997) performed an optical follow-up on 111 late-type stars detected in the RASS south of the Taurus molecular cloud. Among these off-cloud stars 24 have a Li excess and were classified either as PMS (9) or as young ZAMS (15) objects.

Following our works on the EXOSAT sample, we defined a new sample of active cool stars EUV-selected with the ROSAT WFC (Pounds et al. 1993; Pye et al. 1995) and performed spectroscopic (Li I 6708Å, H$\alpha$and Ca II H&K lines) and photometric observations, using the CAT and 50cm ESO telescopes in the South, the McMath telescope at Kitt Peak and the 80cm APT telescope at Catania Observatory in the North.

In this paper we present a subset of these data, that include photometric observations and RV measurements for the stars in our sample that are observable from the southern hemisphere.