RE1016 - 05: a white dwarf binary discovered with the ROSAT Wide Field Camera

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Abstract
We present new data on the extreme-ultraviolet source RE1016 – 05, discovered in the ROSAT Wide Field Camera All-Sky Survey. Spectroscopy of possible optical counterparts reveals an object with a composite DA white dwarf and M dwarf spectrum, only 19 arcsec distant from the Wide Field Camera (WFC) source coordinates. The spectra exhibit hydrogen Balmer line emission in the cores of wide Balmer absorption lines arising from the white dwarf, together with weaker He I line emission. There is evidence for variability in the optical emission-line flux on timescales of hours, but no variation in EUV output is detected. We classify the cool companion in the range dM1–dM3, and determine the white dwarf temperature to be in the range 37 000–47 000 K. The distance modulus derived from the red dwarf data is 5.3 ± 0.8. We discuss possible production mechanisms for the emission-line spectrum and the nature of the system. We also note the presence of a dM0 star only 3 arcsec away from RE1016 – 05, and conclude that it may be a distant third member of the system.

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