A Close Binary Nucleus in the Most Oxygen-Poor Planetary Nebula PN G135.9+55.9

We report Far Ultraviolet Spectroscopic Explorer and new deep optical spectroscopic observations of PN G135.9+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 non-LTE 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 G135.9+55.9 is a close binary. The large semiamplitude 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.