We report the detection of eclipses in the close white-dwarf – M-dwarf binary star RX J2130.6+4710 .
We present lightcurves in the B , V and I bands and fast photometry obtained with the three channel CCD photometer Ultracam of the eclipse in the u ^ { \prime } , g ^ { \prime } and r ^ { \prime } bands .
The depth of the eclipse varies from 3.0 magnitudes in the u ^ { \prime } band to less than 0.1 magnitudes in the I band .
The times of mid-eclipse are given by the ephemeris BJD ( Mid-eclipse ) = 2 452 785.681876 ( 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 is not ideal for spectral classification .
We have compared the spectra of the white dwarf to synthetic spectra from pure hydrogen model atmospheres to estimate that the effective temperature of the white dwarf is \mbox { $T _ { \mbox { \scriptsize eff } } $ } = 18 000 \pm 1000 K. We have used the width of the primary eclipse and duration of totality measured precisely from the Ultracam u ^ { \prime } 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 \pm 0.023 M _ { \odot } and a radius of 0.534 \pm 0.053 R _ { \odot } , which is a typical radius for stars of this mass .
The mass of the white dwarf is 0.554 \pm 0.017 M _ { \odot } and its radius is 0.0137 \pm 0.0014 R _ { \odot } , which is the radius expected for a carbon-oxygen white dwarf of this mass and effective temperature .
The lightcurves 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 lightcurves and radial velocities .
RX J2130.6+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 .