The study of TTauri stars (TTS), first recognized by Ambartsumian (1947) as low-mass pre-main sequence stars, provides insights into the process of star formation and the evolution of stars towards the main sequence. Important issues currently investigated are the frequency of binaries and the distribution of their separations (Leinert et al. 1993; Ghez et al. 1993), which may yield constraints on the models of star formation, as well as the evolution of the angular momentum of TTS (Bouvier et al. 1993a, 1993b).
Another important aspect is the fact that even in well-known star forming regions (SFR) the true number of TTS is quite uncertain, leading to uncertainties in the derivation of the initial mass function (IMF) as well as in the estimates of the efficiency of the star forming process. Moreover, as pointed out by Herbig (1978) there should exist a numerous population of so-called post-TTauri-stars (PTTS), i.e. TTS that have evolved from the population of classical TTS (CTTS) towards the main sequence and therefore should show properties very similar to normal main-sequence stars. As compared to the large expected numbers of PTTS, only few stars that might belong to this population have been identified so far, while many of the known WTTS are more or less coeval with the CTTS.
WTTS often show strong X-ray emission, and therefore are most easily detected in the X-ray band, while due to their lack of strong emission lines in the optical spectral range most of these stars are very difficult to detect by optical objective prisms surveys like those carried out in Lupus by Schwartz (1977) and Thé (1962).
In a previous paper (Wichmann et al. 1996) we have presented results
obtained from a similar survey of the Taurus-Auriga SFR based on the
ROSAT All-Sky-Survey (RASS). Here we present first results
from a similar study of the
Lupus SFR. At a distance of 140pc (Hughes et al. 1993),
Lupus belongs to the nearest nearby
star forming regions. Its
population of 60 TTauri-stars known prior to ROSAT
(Krautter 1991) is
concentrated in four small subgroups (Lupus 1-4).
The mass spectrum of these TTS was found to be quite unusual,
as it exhibits a higher proportion of very low-mass stars
than found in other SFRs (Hughes et al. 1994).
The area of our study covers about
230 square degrees and is located between 
and
in right ascention and
between 
and 
in declination, thus not only
covering all four subgroups,
but extending significantly beyond them.