WHITE-DWARFS OBSERVED BY THE HIPPARCOS SATELLITE

The HIPPARCOS satellite has measured trigonometric parallaxes for 20 w hite 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 paral laxes with the most recent ground-based determinations. From a spectro scopic analysis of new optical observations we determine atmospheric p arameters T-eff and log g. From the angular diameters and the parallax es radii are obtained, and masses are calculated from these radii and the spectroscopic log g. These data are used to test the theoretical m ass-radius relation. On the other hand, assuming this relation, includ ing finite temperature evolutionary effects, we can derive radii and m asses from the effective temperatures and parallaxes. From these we ca lculate surface gravities, which can be compared to the results of the spectroscopic analysis.