White dwarfs observed by the HIPPARCOS satellite

TheHIPPARCOSsatellite hasmeasured trigonometric parallaxes for 20 white dwarfs. With the exception of one each of the spectral types DZ, DC, DB, DQ, the majority are of spectral type DA. We compare the parallaxes with the most recent ground-based determinations. Froma spectroscopic analysis of new optical observations we determine atmospheric parameters Teff and log g. From the angular diameters and the parallaxes radii are obtained, and masses are calculated from these radii and the spectroscopic log g. These data are used to test the theoretical mass-radius relation. On the other hand, assuming this relation, including finite temperature evolutionary effects, we can derive radii and masses from the effective temperatures and parallaxes. From thesewe calculate surface gravities, which can be compared to the results of the spectroscopic analysis.