HD49798/RX J0648.0‐4418 is the only known X‐ray binary composed by a hot sub‐dwarf and an X‐ray pulsar. An XMM‐Newton observation performed in May 2008 has shown that the 13 seconds pulsations 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, it has been possible to accurately measure the masses of the two stars of this post common envelope binary. With a mass of 1.28±0.05M, RX J0648.0‐4418 is one of the most massive white dwarf and the one with the shortest spin period. The future evolution of this binary system will 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 or a millisecond pulsar.

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