2 Observed sample

Our study includes the surrounding area of the Taurus-Auriga SFR, which extends from 2$^{\rm h}$40$^{\rm m}$ to 5$^{\rm h}$40$^{\rm m}$ in right ascension and from 10$^{\circ}$ to 40$^{\circ}$ in declination, with the central core area(4$^{\rm h}<\alpha<$ 5$^{\rm h}$, 15$^{\circ}<\delta<$ 34$^{\circ}$, previously studied by Wichmann et al. 1996) excluded. Within our sample area of about 10³ square degrees, 219 X-ray sources were selected based on the RASS-BSC and the selection criteria developed by Neuhäuser et al. (1995a), $-0.15\leq {\rm HR1} \leq 1$ and $-0.3\leq{\rm HR2}\leq0.5$ where the HRs represent hardness ratios computed with count rates of five different energy bands of the ROSAT detection. The X-ray sources thus selected all, except one, have optical counterparts in the Digital Sky Survey(DSS) within the RASS error box of 20''; their E magnitudes have been estimated from the DSS with a routine developed by Cao et al. (1997) with an accuracy of 0.5 mag, and a sample of 164 X-ray sources with at least one counterpart brighter than E=16 was obtained. Two X-ray sources, IRXS J033310.7+103613 and 1RXS J053434.7+100710, though previously identified to be WTTS candidates by Neuhäuser et al. (1995c) and Alcalá et al. (1996) respectively, have been included in our study. Low-resolution spectroscopy of a list of 175 optical counterparts to 164 ROSAT sources was performed, the V magnitude of each object was estimated at 5570 Å (accurate to 0.2 V) and the spectral type was determined visually. As a result, 19 early type stars and galaxies were omitted and a sub-sample of 156 optical counterparts, associated with 147 X-ray sources that have spectral type later than mid-F and V₅₅₇₀ ${\leq}$ 16 (with one exception of V₅₅₇₀ = 16.3) have been followed up with intermediate-resolution spectroscopic observations.