HS 2237+8154: On the onset of mass transfer or entering the period gap?

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Abstract
We report follow-up observations of a new white dwarf/red dwarf binary HS 2237+8154, identified as a blue variable star from the Hamburg Quasar Survey. Ellipsoidal modulation observed in the R band as well as the radial velocity variations measured from time-resolved spectroscopy determine the orbital period to be Porb ± 0.08 min. The optical spectrum of HS 2237+8154 is well described by a combination of a ± 1500 K white dwarf (assuming ) and a dM 3.5 ± 0.5 secondary star. The distance implied from the flux scaling factors of both stellar components is ± 25 pc. Combining the constraints obtained from the radial velocity of the secondary and from the ellipsoidal modulation, we derive a binary inclination of and stellar masses of and . All observations imply that the secondary star must be nearly Roche-lobe filling. Consequently, HS 2237+8154 may be either a pre-cataclysmic variable close to the start of mass transfer, or – considering its orbital period – a cataclysmic variable that terminated mass transfer and entered the period gap, or a hibernating nova.
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