1 Introduction

T Tauri stars (TTS) can be defined as low-mass (up to 3 $M_{\odot}$), late spectral type (later than mid-F) and variable pre-main sequence stars with relatively young age (from 10⁵ yrs to a few 10⁷ yrs). Weak-line T Tauri stars (WTTS), unlike classical T Tauri stars (CTTS) which are characterized by strong optical emission lines, ultraviolet and infrared excesses, are believed to be relatively devoid of circumstellar matter. Taurus-Auriga is one of the best studied ongoing low-mass Star Forming Regions (SFR) with a distance of $\sim$140 pc. About 150 TTS were known prior to the ROSAT mission in the surrounding area of the T association, most of which are listed in the Herbig-Bell catalog (Herbig & Bell 1988), and nearly all CTTS have been previously discovered by their conspicuous optical characteristics. Since WTTS are intrinsically more X-ray luminous than CTTS (Neuhäuser et al. 1995a) and thus are much easier detected by the spatially unbiased ROSAT All Sky Survey (RASS), the advent of the RASS offers for the first time the opportunity to extend the search for X-ray active WTTS in nearby SFRs, with a flux limit comparable with typical EO pointed observations. So far, $\sim$70 previously unknown WTTS candidates have been uncovered in the central part of the Taurus-Auriga SFR (Wichmann et al. 1996). Furthermore, 15 WTTS candidates (Neuhäuser et al. 1995c) and later 30 more WTTS candidates (Magazzú et al. 1997) have been identified to the south of the Taurus molecular clouds on the basis of RASS data and follow-up optical observations. A few TTS candidates have also been discovered sporadically in deep ROSAT pointed observations in Taurus-Auriga (Strom & Strom 1994; Carkner et al. 1996; Wichmann et al. 1996).

The release of the RASS Bright Source Catalog (RASS-BSC) (Voges et al. 1996) has enabled us to study the surroundings of Taurus-Auriga to reveal more WTTS. A complete sample of X-ray sources selected from RASS-BSC were identified using ground-based optical follow-up observations. Low-resolution spectroscopic observations have been carried out to discriminate late type optical counterparts from early type stars and galaxies in the error box of the RASS, and to estimate their visual magnitudes at the time of observation. Intermediate-resolution spectroscopy of the late type stars then followed to investigate their proper spectral types and to measure H$_{\alpha}$ and LiI equivalent widths. The identification of the RASS sources and the discovery of new WTTS candidates in the Taurus-Auriga SFR are concerned in this paper.