* Astronomy

Title: The shortest period detached white dwarf + main-sequence binary
Authors: S. G. Parsons, T. R. Marsh, B. T. Gänsicke, V. S. Dhillon, C. M. Copperwheat, S. P. Littlefair, S. Pyrzas, A. J. Drake, D. Koester, M. R. Schreiber, A. Rebassa-Mansergas

We present high-speed ULTRACAM and SOFI photometry and X-shooter spectroscopy of the recently discovered 94 minute orbital period eclipsing white dwarf / main-sequence binary SDSS J085746.18+034255.3 (CSS 03170) and use these observations to measure the system parameters. We detect a shallow secondary eclipse and hence are able to determine an orbital inclination of 85.5 ±0.2 deg. The white dwarf has a mass of 0.51 ±0.05 Solar masses and a radius of 0.0247 ±0.0008 solar radii. With a temperature of 35,300 ±400K the white dwarf is highly over-inflated if it has a carbon-oxygen core, however if it has a helium core then its mass and radius are consistent with evolutionary models. Therefore, the white dwarf in SDSS J085746.18+034255.3 is most likely a helium core white dwarf with a mass close to the upper limit expected from evolution. The main-sequence star is an M8 dwarf with a mass of 0.09 ±0.01 Solar masses and a radius of 0.110 ±0.004 solar radii placing it close to the hydrogen burning limit. The system emerged from a common envelope ~20 million years ago and will reach a semi-detached configuration in ~400 million years, becoming a cataclysmic variable with a period of 66 minutes, below the period minimum.

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