RX J2130.6+4710 - an eclipsing white dwarf - M-dwarf binary star

Abstract

(Abridged) We report the detection of eclipses in the close white-dwarf - M-dwarf binary star RXJ2130.6+4710. We estimate that the spectral type of the M-dwarf is M3.5Ve or M4Ve. We estimate that the effective temperature of the white dwarf is Teff = 18000K +- 1000K. We have used the width of the primary eclipse and duration of totality measured precisely from Ultracam u' data combined with the amplitude of the ellipsoidal effect in the I band and the semi-amplitudes of the spectroscopic orbits to derive masses and radii for the M-dwarf and white dwarf. The M-dwarf has a mass of 0.555 +- 0.023 solar masses and a radius of 0.534 +- 0.053 solar radii, which is a typical radius for stars of this mass. The mass of the white dwarf is 0.554 +- 0.017 solar masses and its radius is 0.0137 +- 0.0014 solar radii, which is the radius expected for a carbon-oxygen white dwarf of this mass and effective temperature RXJ2130.6+4710 is a rare example of a pre-cataclysmic variable star which will start mass transfer at a period above the period gap for cataclysmic variables.