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We report the results of XMM-Newton observations of HD49798/RXJ0648.0-4418,the only known X-ray binary consisting of a hot sub-dwarf and a white dwarf.The white dwarf rotates very rapidly (P=13.2 s) and has a dynamically measuredmass of 1.28+/-0.05 M_sun. Its X-ray emission consists of a strongly pulsed,soft component, well fit by a blackbody with kT~40 eV, accounting for most ofthe luminosity, and a fainter hard power-law component (photon index ~1.6). Aluminosity of ~10^{32} erg/s is produced by accretion onto the white dwarf ofthe helium-rich matter from the wind of the companion, which is one of the fewhot sub-dwarfs showing evidence of mass-loss. A search for optical pulsationsat the South African Astronomical Observatory 1.9-m telescope gave negativeresults. X-rays were detected also during the white dwarf eclipse. Thisemission, with luminosity 2x10^{30} erg/s, can be attributed to HD 49798 andrepresents the first detection of a hot sub-dwarf star in the X-ray band.HD49798/RXJ0648.0-4418 is a post-common envelope binary which most likelyoriginated from a pair of stars with masses ~8-10 M_sun. After the currentHe-burning phase, HD 49798 will expand and reach the Roche-lobe, causing ahigher accretion rate onto the white dwarf which can reach the Chandrasekharlimit. Considering the fast spin of the white dwarf, this could lead to theformation of a millisecond pulsar. Alternatively, this system could be a TypeIa supernova progenitor with the appealing characteristic of a short timedelay, being the descendent of relatively massive stars.

X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418