Aims. The ESO SN Ia Progenitor Survey (SPY) aims at finding merging doubledegenerate binaries as candidates for supernova type Ia (SN Ia) explosions. Awhite dwarf merger has also been suggested to explain the formation of raretypes of stars like R CrB, extreme helium or He sdO stars. Here we present thehot subdwarf B binary GD 687, which will merge in less than a Hubble time. Methods. The orbital parameters of the close binary have been determined fromtime resolved spectroscopy. Since GD 687 is a single-lined binary, the spectracontain only information about the subdwarf primary and its orbit. From highresolution spectra the projected rotational velocity was derived. Assumingorbital synchronisation, the inclination of the system and the mass of theunseen companion were constrained. Results. The derived inclination is $i=39.3^{+6.2}_{-5.6}\,^{\circ}$. Themass $M_{\rm 2}=0.71_{-0.21}^{+0.22}\,M_{\rm \odot}$ indicates that thecompanion must be a white dwarf, most likely of C/O composition. This is onlythe fourth case that an sdB companion has been proven to be a white dwarfunambiguously. Its mass is somewhat larger than the average white dwarf mass,but may be as high as $0.93\,M_{\rm \odot}$ in which case the total mass of thesystem comes close to the Chandrasekhar limit. Conclusions. GD 687 will evolve into a double degenerate system and merge toform a rare supermassive white dwarf with a mass in excess of solar. A death ina sub-Chandrasekhar supernova is also conceivable.
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