  An XMM-Mewton observation performed in May 2008 has confirmed that the 13
seconds pulsations in the X-ray binary HD 49798/RX J0648.0-4418 are due to a
rapidly rotating white dwarf. From the pulse time delays induced by the 1.55
days orbital motion, and the system's inclination, constrained by the duration
of the X-ray eclipse discovered in this observation, we could derive a mass of
1.28+/-0.05 M_sun for the white dwarf. The future evolution of this post common
envelope binary system will likely involve a new phase of mass accretion
through Roche-lobe overflow that could drive the already massive white dwarf
above the Chandrasekhar limit and produce a Type Ia supernova.
