The study of T Tauri stars (TTS) is a main step towards the
understanding of star formation in the Galaxy and early phases of
low-mass stellar evolution. Early definitions (e.g. Herbig
1962; Bastian et al. 1983) describe TTS as low-mass
pre-main sequence (PMS) objects showing in their spectrum emission from the
hydrogen Balmer lines and the Ca II H and K lines, reflecting the fact
that TTS were discovered in 
surveys of nearby molecular clouds
("classical'' TTS, cTTS).
After the Einstein Observatory (EO) X-ray mission other objects have
joined the T Tauri family, in particular X-ray active "weak-line'' TTS
(wTTS), which lack strong emission lines (
() 
) (e.g. Gahm 1980; Feigelson & DeCampli
1981) and, usually, near-infrared excess emission (Walter
1986). From X-ray emission variability and the spatial incompletness
of EO pointed observations, Walter et al. (1988) deduced that
there should be as many as 1000 wTTS in the general Taurus-Auriga region,
many of which can be discovered with the RASS. The population defined by the
new wTTS discovered by Walter et al. (1988) by optical
follow-up observations of previously unidentified EO sources cannot be
identified with the post-TTS population proposed by Herbig
(1978), as many wTTS share the same locus in the H-R diagram as
cTTS. In the optical spectrum of all TTS the LiI
absorption doublet is a
prominent feature indicative of their PMS nature (Magazzù et al.\
1992; Martín et al.\
1994). The majority of TTS have been discovered in areas where
molecular gas has been detected. One of the best studied areas is the
Taurus-Auriga star forming region (SFR), a T association at a distance of
pc (Elias 1978; Kenyon et al. 1994).
The advent of the ROSAT All-Sky Survey (RASS) has enabled us to extend the search for X-ray active low-mass stars to the complete sky, with a flux limit comparable with typical EO pointed observations. By studying X-ray spectra of RASS-detected well-known TTS and unidentified RASS sources in the Taurus-Auriga SFR, Neuhäuser et al.\ (1995a) have proposed that several hundreds of coronally active TTS are hidden in the RASS database. So far, 76 PMS stars have been discovered in the central parts of the Taurus-Auriga association (Wichmann et al.\ 1996). Some other PMS stars have been discovered with optical follow-up observations of sources found in deep ROSAT pointed observations in Taurus-Auriga (Strom & Strom 1994; Carkner et al.\ 1996; Wichmann et al. 1996). Based on ROSAT observations, many new PMS stars have been discovered in other SFRs as well (see Alcalá et al. 1995; Neuhäuser 1996; Krautter 1996).
As the set of unidentified RASS sources is very large, Sterzik et al. (1995) proposed an efficient way of pre-selecting TTS candidates just from ROSAT and Hubble Space Telescope Guide Star Catalog (GSC) data alone, which are easily accessible for all unidentified RASS sources having a near-by GSC counterpart. Surprisingly, Sterzik et al. (1995) did not find any gradient in the space density of TTS candidates at the edges of the Orion molecular clouds and they proposed to extend the search for TTS even outside the cloud complexes. The strength of RASS as being spatially unbiased allows us to survey for TTS also outside the regions previously known to be populated by TTS.
In this paper, we report on the results of such a study conducted south of the Taurus-Auriga clouds. In Sect. 2, we describe ROSAT observations and X-ray data analysis. The method introduced by Sterzik et al. (1995) has been then applied to select TTS candidates (Sect. 3). Optical follow-up observations of these TTS candidates and data reduction are explained in Sect. 4, results are presented in Sect. 5 and discussed in the last section.
In addition to data presented here, high-resolution spectral observations have been obtained for most of our stars to study their radial velocity; these data as well as proper motions of several new PMS stars are given in Neuhäuser et al. (1997), in which the kinematics and possible modes of origin of newly discovered PMS stars are discussed. Preliminary results of our study were reported by Neuhäuser et al. (1995c), who discussed the results of optical observations of 15 RASS-selected TTS candidates south of Taurus-Auriga. These objects are included in the sample discussed here.