Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images. I. Time variability and spectral classification of eight southern stars.

As part of a larger program to study the optical properties of serendipitous Exosat sources, optical photometry and low-resolution spectroscopy are presented for eight southern stars which have been identified as optical counterparts of Exosat sources. X-ray flux variability has been detected for three of them. In all three cases the optical counterparts show RS CVn-type variability. Of the remaining five stars, two are found to be variable in the optical. The high X-ray luminosities inferred from the derived spectroscopic parallaxes indicates that these variable sources are all very active systems, possibly RS CVn binaries. The three nonvariable sources are more likely normal main-sequences stars, two of them with a rather high level of coronal emission.