X-RAY OPTICAL OBSERVATIONS OF STARS WITH SHALLOW CONVECTION ZONES (A8-G2V)/

We present Walraven photometry and ROSAT All-Sky Survey data for a sam ple of 173 bright main-sequence stars with spectral types between A8V and G2V. These observations are part of a study of the onset of magnet ic surface activity along the main sequence. Values for the effective temperature, surface gravity and interstellar reddening have been obta ined from a comparison of the observed Walraven colours with theoretic al values. These parameters have been used to derive accurate X-ray su rface flux densities.