Astron. Astrophys. 324, L25-L28 (1997)

1. Introduction

V471 Tau is among the best studied eclipsing binary systems. It comprises a K2 main sequence star and a hot white dwarf. System parameters suggest that the detached stars form a post-common envelope WD MS binary (see e.g. de Kool & Ritter 1993, and references therein). Hence it is an object which is important for our understanding of close binary evolution.

Based on its proper motion it is commonly anticipated that V471 Tau is a member of the Hyades cluster which, however, was at variance with several parallax measurements. Consequently, V471 Tau was excluded from population studies of the Hyades (Eggen & Iben 1988, Weidemann et al. 1992). The question as to the Hyades membership of V471 Tau is not only important for these studies, but also for the analysis of the binary system in itself, as it is desirable to know its distance. Recently published ground-based as well as HIPPARCOS parallaxes have now confirmed the cluster membership of V471 Tau. The HIPPARCOS result ( mas, Provencal et al. 1996) agrees with the recently published ground-based value ( mas, Van Altena et al. 1995).

The V471 Tau binary system is important for white dwarf research because it allows to study in great detail the characteristics of an individual WD. In particular we are interested in the WD mass and radius in order to compare with the theoretically predicted mass-radius relation for white dwarfs. Only very few mass determinations exist which are precise enough to test this relation, and the most reliable ones were derived from white dwarfs in binaries (see e.g. Schmidt 1996). Radius estimates from spectroscopically determined are usually hampered by poorly known distances, although the situation has been improved considerably by recent HIPPARCOS results for 20 WDs (Vauclair et al. 1997). Hence white dwarfs in binaries play an important role as the distance may be found from a luminosity estimate for the companion. Unfortunately, in most such cases the late type companion dominates at optical wavelengths so that the WD is not accessible for spectroscopic analyses. Measurements in the ultraviolet, where the WD radiation dominates, are necessary for further investigations.

In the case of V471 Tau Guinan & Sion (1984) used IUE low resolution spectra to derive =35 000 3 000 K (and 8) from a fit to the Ly profile, which is marginally in agreement with estimates from light curve studies of the system ( =31 000 2 000 K, Cester & Pucillo 1976; =32 000 K, Young & Nelson 1972). In this paper we present a new and more precise spectroscopic temperature and gravity determination which is based on the hydrogen Lyman line series covered by ORFEUS, combined with pure hydrogen line-blanketed model atmospheres.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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