Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images IV. The northern hemisphere sample

We present high-precision photometry, high- and medium-resolution spectrosc opy for a sample of 32 stars likely to be the optical counterparts of X-ray sources serendipitously detected by the EXOSAT satellite. Using also recen t results from the Hipparcos satellite, we infer spectral types, compute X- ray luminosities and Li abundances and investigate the single or binary nat ure of the sample stars. We found eleven new variable stars, whose photomet ric periods fall in the 1.2-27.5 day range, for most of which the optical v ariability is consistent with the presence of photospheric cool spots. For our sample of X-ray selected stars we confirm the existence of a strong cor relation between the stellar rotation rate and the level of activity, and a lso between the X-ray and bolometric luminosities. Two stars in our sample are likely to be pre-main sequence objects, one is likely to be a previousl y unknown M-type star within 25 pc.