X-ray/optical observations of stars with shallow convection zones ^*

¹ Astronomical Institute "Anton Pannekoek" / CHEAF, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands
² Physics Department UAH, Huntsville, AL 35899, U.S.A.
³ Max Planck Institut für Extraterrestrische Physik, Giessenbachstraße 1, 8046 Garching bei München, Germany

Send offprint request to: J. van Paradijs

Received: 3 February 1997
Accepted: 16 June 1997

Abstract

We present Walraven photometry and ROSAT All-Sky Survey data for a sample of 173 bright main-sequence stars with spectral types between A8V and G2V. These observations are part of a study of the onset of magnetic surface activity along the main sequence. Values for the effective temperature, surface gravity and interstellar reddening have been obtained from a comparison of the observed Walraven colours with theoretical values. These parameters have been used to derive accurate surface flux densities.