1994A&A...285..272T

We present high-resolution spectra in the Li I 6708 A region, and multicolor photometry, for a sample of active stars detected serendipitously by the EXOSAT satellite. We analyse the spectral data with a spectral synthesis code and derive Li abundances, rotation rates and metallicities. We use our multicolor photometry to infer the spectral classification and distance of the stars in the sample. The derived Li abundances are compared with those of various samples including randomly selected field stars, RS CVn binaries, and clusters and associations of different ages. We show that Li per se is not sufficient to determine unambiguously the nature and evolutionary state of the sample stars, but can provide useful constraints if used in conjunction with other diagnostics like binarity, spectral class, rotation rate and X-ray luminosity. We show in particular that at least one third of the EXOSAT sample is constituted by young stars, with ages comparable to or younger than the Pleiades (∼7x10⁷ yr). Another third consists of RS CVn binaries, while the classification of the remaining objects is uncertain. They could be either young stars or evolved active binaries. We argue that a similar composition also applies to the sample of X-ray stellar sources detected serendipitously by the Einstein Observatory.