TESTING THE WHITE-DWARF MASS-RADIUS RELATION WITH HIPPARCOS

We present the Hipparcos parallaxes and resulting radii for 10 white d warfs in visual binaries or common proper-motion systems and 11 held w hite dwarfs. For bright stars, Hipparcos parallaxes have uncertainties approaching 1 mas and are thus considerably more accurate than earlie r ground-based parallaxes. Overall, our results support the prediction s of the white dwarf mass-radius relation and our understanding of ste llar degeneracy. Our most important finding for an individual object i s the position of 40 Eri B, a well-known puzzle, now consistent with s ingle-star evolution. In addition, we present evidence supporting the existence of a range of atmosphere thicknesses for hydrogen (DA) white dwarfs.