Testing the white dwarf mass-radius relation with Hipparcos

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