Empirical luminosities and radii of early-type stars after Hipparcos

Using Hipparcos parallaxes we derive empirical luminosities and radii of th e early-type stars for which the effective temperatures are known from dire ctly measured angular diameters and total absolute fluxes. The empirical lu minosities allow a direct comparison of the position of these stars in the fundamental HR diagram with evolutionary tracks. The comparison shows an ov erall agreement with the Y = 0.30 and Z = 0.02 tracks computed with OPAL op acities and moderate amount of overshooting from the convective core. In ad dition, we present evidence that systematic errors of the masses read off t he evolutionary tracks are below 10%. Consequently, the surface gravities o btained from these "evolutionary" masses and the empirical radii are very n early model-independent. Spectrographic and photometric observations of the se stars can therefore be used for verifying model atmospheres and calibrat ing photometric log g indices.