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The X-ray source RX J0648.0-4418 is the only confirmed binary system in whicha compact object, most likely a massive white dwarf, accretes from a hotsubdwarf companion, the bright sdO star HD 49798. The X-ray emission from thissystem is characterized by two periodic modulations caused by an eclipse, atthe orbital period of 1.55 d, and by the rotation of the compact object with aspin period of 13.2 s. In 2011 we obtained six short XMM-Newton observationscentered at orbital phase 0.75, in order to study the system during theeclipse, and spaced at increasingly long time intervals in order to obtain anaccurate measure of the spin-period evolution through phase-connected timing.The duration of the eclipse ingress and egress, 500 s, indicates the presenceof an X-ray emitting region with dimensions of the order of a few 10^4 km,surrounding the pulsar and probably due to scattering in the companion's wind.We derived an upper limit on the spin-period derivative |Pdot|<6x10^-15 s/s,more than two orders of magnitude smaller than the previously available value.Significant X-ray emission is detected also during the 1.2 hours-long eclipse,with a luminosity of about 3x10^30 erg/s. The eclipse spectrum shows prominentemission lines of H- and He-like nitrogen, an overabundant element in HD 49798.These findings support the suggestion that the X-ray emission observed duringthe eclipse originates in HD 49798 and that the processes responsible for X-rayemission in the stellar winds of massive O stars are also at work in the muchweaker winds of hot subdwarfs.

X-ray emission from the luminous O-type subdwarf HD 49798 and its compact companion