1 Introduction

With the launch of the first imager X-ray satellite, Einstein, X-ray emission was detected from stars of nearly all spectral types (see for example Vaiana et al. 1981; Pallavicini 1989). In addition to the data obtained by using pointed observations, a large body of information on the coronae of late-type stars came from the study of serendipitously detected stars, both in the Einstein EMSS and the EXOSAT surveys. Results on stellar coronae are summarized in numerous reviews (i.e. Pallavicini 1989; Vaiana 1990; Linsky 1990). Late-type stars show a large spread in their X-ray luminosities (up to 3 orders of magnitudes), which correlate with stellar rotational velocities ($L_{\rm X} \propto v \sin i^2$, Pallavicini et al. 1981).

Up to now, the largest sample of X-ray selected late-type stars came from the EMSS survey. This sample consists of a total of 169 solar type stars, out of which 128 were studied by Fleming (1988, also Fleming et al. 1989). Fleming's results confirm the correlation between X-ray luminosity and rotational velocity for solar-type single stars, although he finds a different correlation ($L_{\rm X} \propto v \sin i$) to the one found by Pallavicini et al. (1981). He also found evidence for a saturation of the X-ray emission. This saturation would represent a maximum level of X-ray emission for solar-type stars, corresponding to a surface flux of $F_{\rm X} \sim 7 \ 10^7\ {\rm erg} \ {\rm s}^{-1} \ {\rm cm}^{-2}$.

Since the launch of the ROSAT satellite (Trümper 1992) with its all-sky survey (RASS), larger samples are available. The ROSAT PSPC passband, 0.1 - 2.4 keV, is different from the Einstein IPC (0.3 - 3.5 keV). It is therefore necessary to investigate the nature of the coronal sources in the RASS and to compare their characteristics with those from the pre-ROSAT era.

In this work we present the results of photometric observations of solar-type stars detected in the RASS. Spectral classification obtained from the colours is presented, as well as the distance and X-ray luminosity estimates derived from this classification. Our results are compared with those obtained from the Einstein and EXOSAT surveys. We finally aim at defining selection criteria for solar-type stars, in order to allow automatic searches of the RASS database.