MASS AND RADIUS OF THE WHITE-DWARF IN THE BINARY V471-TAU FROM ORFEUSAND HIPPARCOS OBSERVATIONS

We present a model atmosphere analysis of the white dwarf component in the K2V/DA eclipsing binary system V471 Tau, based upon FUV spectra o btained by the ORFEUS satellite. This system is regarded as the protot ype of a post-common envelope WD + MS binary and, hence, is of conside rable importance for our understanding of close binary evolution. From a line profile fit to the FUV hydrogen Lyman series we obtain T-eff = 35 125 K and log g = 8.21. The HIPPARCOS trigonometric parallax measu rement (21.37 +/- 1.62 mas) confirms the Hyades membership of V471 Tau and we can derive the white dwarf radius. Together with its astrometr ic mass, which is known from analyses of the orbital parameters and wh ich is compatible with our spectroscopic surface gravity determination , we can compare this individual WD to the theoretical mass-radius rel ation.