The DA plus dMe eclipsing binary EC13471-1258: its cup runneth over ... just

The optical spectrum and light curve of EC13471-1258 show that it is an eclipsing binary with an orbital period of 3(h) 37(m) comprising a DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf are observed lasting 14 min, with the partial phases lasting 54 s. On one occasion, two pre-eclipse dips were seen. Timings of the eclipses over 10 yr show jitter of up to 12 s. Flares from the M dwarf are regularly observed. The M dwarf also shows a large-amplitude ellipsoidal modulation in the V-band light curve. The component stars emit almost equal amounts of light at 5500 Angstrom. Hubble Space Telescope (HST) STIS spectra show strong Lyman-alpha absorption with weak metal lines of C I, II and Si II superimposed. Model atmosphere analysis yielded an effective temperature of 14 220 +/- 300 K and log g of 8.34 +/- 0.20 for the white dwarf with these errors being strongly correlated. Its metal abundance is 1/30th solar with an uncertainty of 0.5 dex, and it is rapidly rotating with V-1 sin i = 400 +/- 100 km s(-1). The white dwarf also shows radial velocity variations with a semi-amplitude of 138 +/- 10 km s(-1). The gravitational redshift of the white dwarf was measured as 62 km s(-1). From optical spectroscopy the spectral type of the M dwarf was found to be M3.5-M4, its temperature 3100 +/- 75 K, its rotational velocity 140 +/- 10 km s(-1), its radial velocity semiamplitude 266 +/- 5 km s(-1), its mean V - I colour 2.86 and its absolute V magnitude 11.82. Intriguingly, its metal abundance is normal solar. The Halpha emission line shows at least two distinct components, one of which is uniformly distributed around the centre of mass of the Mdwarf and provided the estimate of the rotational velocity of the M dwarf. The other arises from the other side of the binary centre of mass, well within the white dwarf Roche lobe. This behaviour is confirmed by Doppler tomography, which shows the presence of two distinct velocity components within the primary Roche lobe. The interpretation of these features is uncertain. Variations in strength of the components with binary phase can be attributed to optical thickness in the Balmer lines. Similar behaviour is seen in the observations of the other Balmer emission lines, although with a poorer signal-to-noise ratio. Flares in H were observed and are consistent with arising from the vicinity of the M dwarf. Dynamical solutions for the binary are discussed and yield an inclination of 75.5 +/- 2 0degrees,a white dwarf mass and radius of 0 78 +/- 0 04 M-. and 0.011 +/- 0.01 R-., and an M dwarf mass and radius of 0.43 +/- 0.04 M-. and 0.42 +/- 0.02 R-.. These parameters are consistent with the Wood mass-radius relation for white dwarfs and the Clemens et al. mass-radius relation for M dwarfs; we argue that the M dwarf just fills its Roche lobe. The radius of the white dwarf and the model fit imply a distance of 48 +/- 5 pc and an absolute V magnitude of 11.74. The rapid rotation of the white dwarf strongly suggests that the system has undergone mass transfer in the past, and implies that it is a hibernating cataclysmic variable. The M dwarf shows the properties expected of secondaries in cataclysmic variables: chromospheric activity and angular momentum loss.