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

We report the detection of eclipses in the close white-dwarf–M-dwarf binary star RX J2130.6+4710. We present light curves in the B, V and I bands and fast photometry obtained with the three-channel CCD photometer Ultracam of the eclipse in the u′, g′ and r′ bands. The depth of the eclipse varies from 3.0 mag in the u′ band to less than 0.1 mag in the I band. The times of mid-eclipse are given by the ephemeris BJD(mid-eclipse) = 2 452 785.681 876(2) + 0.521 035 625(3) E, where figures in parentheses denote uncertainties in the final digit. We present medium-resolution spectroscopy from which we have measured the spectroscopic orbits of the M dwarf and white dwarf. We estimate that the spectral type of the M dwarf is M3.5Ve or M4Ve, although the data on which this is based are not ideal for spectral classification. We have compared the spectra of the white dwarf with synthetic spectra from pure hydrogen model atmospheres to estimate that the effective temperature of the white dwarf is T_eff = 18 000 ± 1000 K. We have used the width of the primary eclipse and duration of totality measured precisely from the 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 M⊙ and a radius of 0.534 ± 0.053 R⊙, which is a typical radius for stars of this mass. The mass of the white dwarf is 0.554 ± 0.017 M⊙ and its radius is 0.0137 ± 0.0014 R⊙, which is the radius expected for a carbon–oxygen white dwarf of this mass and effective temperature. The light curves are affected by frequent flares from the M dwarf and the associated dark spots on its surface can be detected from the distortions to the light curves and radial velocities. RX J2130.6+4710 is a rare example of a pre-cataclysmic variable star that will start mass transfer at a period above the period gap for cataclysmic variables.