  We report FUSE and new deep optical spectroscopic observations of PN G
135.9+55.9, the most oxygen-poor planetary nebula located in the Galactic halo.
These observations allow us to estimate the gravity of the central star by
fitting the profile of the observed H I absorption lines with NLTE model
atmospheres. Our best fit implies that the central star is still in a pre-white
dwarf stage. We also find large variability of the radial velocities of the
absorption component of the Balmer lines on a timescale of hours. This is
direct evidence that the nucleus of PN G 135.9+55.9 is a close binary. The
large semi-amplitude of the radial velocity variations and the probably short
period suggest a massive compact companion, likely a white dwarf. Although our
orbital solutions are very preliminary, they indicate that the total mass of
the system probably exceeds the Chandrasekhar limit. If confirmed, this would
make this binary the potential progenitor of a type Ia supernova.
