1 Introduction

X-ray emission from late-type stars is generally attributed to magnetically heated stellar coronae. As suggested by the example of the Sun's corona, the emitting hot (> 10⁶ K) plasma is believed to be confined by coronal magnetic fields, which ultimately originate from the interaction between rotation and outer convection zones. The solar X-ray luminosity varies approximately between $3 \ 10^{25}$ and $1\ 10^{27}$ ergs^-1 (extrapolated to a 1-300 Å "bolometric X-ray band''; cf. Haisch & Schmitt 1996; Acton 1996) during the solar cycle and is known to be strongly correlated with other magnetic activiy indicators (e.g., sunspot numbers, flare frequency, chromospheric CaII emission). Hence, it can be regarded as a good activity indicator also for other stars.

In principle, any late-type star should be able to sustain a corona, and henceforth, be an X-ray source. Indeed, already the Einstein Observatory detected X-ray emission from a large number of late-type stars (cf. Vaiana et al. 1981; Maggio et al. 1987). Detailed studies of the immediate solar environment (Schmitt et al. 1995; Schmitt 1997) revealed that virtually every late-type dwarf star with spectral type later than A7 can be detected as an X-ray source given data of sufficient sensitivity; Schmitt (1997) argues that late-type stars emit X-rays at least at a level such that the apparent X-ray surface flux $F_{\rm x}$ exceeds $\approx$10⁴ ergcm^-2 s^-1. On the other hand, stellar X-ray luminosities appear to be bounded above by the so-called saturation limit $L_{\rm x}$/$L_{\rm bol} \sim 10^{-3}$, which can be observed both for field stars and stars in open clusters, where one observes that the spectral type above which stars appear to be saturated moves toward later types with increasing age. This is probably related to the angular momentum evolution of young late-type main-sequence stars, which are spun down by magnetic breaking during their first $\sim\!10^8$ yrs on the main-sequence. Enhanced activity can, however, last on much longer timescales, as indicated by example of the Hyades cluster.

Until now, the ROSAT observatory has undertaken the only sensitive X-ray survey (RASS) of the whole sky. These data provide a flux-limited but otherwise unbiased sample of X-ray sources (Voges et al. 1996a). Of the $\approx 1.5 \ 10^5$ detected sources, about one third are considered to be coronal. Many of these coronal X-ray sources have optically faint counterparts which require optical follow-up observations. A smaller number of X-ray sources have bright optical counterparts, and we have systematically searched for X-ray emission from these optically bright stars. The results of our survey are presented as catalogs of X-ray data, which have already been published for OB stars (Berghöfer et al. 1996) and late-type giants and supergiants (Hünsch et al. 1998; hereafter HSV98).