  HD49798/RXJ0648.0-4418 is the only known X-ray binary composed of a hot
subdwarf and a massive white dwarf (M=1.28+/-0.05 Msun). This system, with an
orbital period of 1.55 days, is the outcome of a common envelope evolution,
most likely of a pair of stars with initial masses of about 8-10 Msun. When the
hot subdwarf, currently in a He-burning phase, will expand again and fill its
Roche-lobe, the enhanced mass transfer can rapidly bring the already massive
white dwarf above the Chandrasekhar limit. The possible final fate, either a
Type Ia supernova explosion or an accretion induced collapse, is particularly
interesting in view of the high rotational velocity of this star, which has the
shortest spin period (13 s) observed in a white dwarf.
