Spectroscopic binaries in a sample of ROSAT X-ray sources south of the Taurus molecular clouds

We report the results of our radial-velocity monitoring of spectroscopic binary systems in a sample of X-ray sources from the ROSAT All Sky Survey south of the Taurus-Auriga star-forming region. The original sample of ∼120 sources by Neuhäuser et al. was selected on the basis of their X-ray properties and the visual magnitude of the nearest optical counterpart, in such a way as to promote the inclusion of young objects. Roughly 20% of those sources have previously been confirmed to be very young. We focus here on the subset of the original sample that shows variable radial velocities (43 objects), a few of which have also been flagged previously as being young. New spectroscopic orbits are presented for 42 of those systems. Two of the binaries, RXJ0528.9+1046 and RXJ0529.3+1210, are indeed weak-lined T Tauri stars likely to be associated with the λ Ori region. Most of the other binaries are active objects of the RS CVn-type, including several W UMa and Algol systems. We detect a strong excess of short-period binaries compared to the field, and an unusually large fraction of double-lined systems. This, along with the overall high frequency of binaries out of the original sample of ∼120 sources, can be understood as a selection effect since all these properties tend to favor the inclusion of the objects in a flux-limited X-ray survey such as this by making them brighter in X-rays. A short description of the physical properties of each binary is provided,