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

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 ill the R band as well as the Halpha radial velocity variations measured from time-resolved spectroscopy determine the orbital period to be P-orb = 178.10 +/- 0.08 min. The optical spectrum of HS 2237+8154 is well described by a combination of a T-eff = 11500 +/- 1500 K white dwarf (assuming log g = 8) and a dM 3.5 +/- 0.5 secondary star. The distance implied from the flux scaling factors of both stellar components is d = 105 +/- 25 pc. Combining the constraints obtained from the radial velocity of the secondary and from the ellipsoidal modulation, we derive a binary inclination of i similar or equal to 50degrees-70degrees and stellar masses of M-wd = 0.47-0.67 M-. and M-sec = 0.2-0.4 M-.. 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.