1997A&A...317..815B

An analysis of the observed properties of the sub-dwarf O6 close binary system HD 49798 and its 13.18s ultrasoft X-ray pulsating companion (WGA J0648.0-4418) is presented. On evolutionary grounds we show that the subdwarf must have a degenerate CO core and is in the phase of shell helium burning, which explains its high luminosity. The subdwarf, which probably has a mass between 0.7-1.3M_☉, is the descendant of a massive asymptotic giant branch star that lost its hydrogen-rich outer layers in a common-envelope event. We show that all observations are consistent with the X-ray source being a weakly magnetized massive white dwarf which is accreting matter from the wind of its subdwarf companion. We exclude a neutron star companion on the ground of (1) the ultrasoft spectrum of the X-ray source; (2) the very close resemblance of the X-ray spectrum and luminosity with that of the soft intermediate polars; (3) the relative closeness of HD 49798, which implies a Galactic birthrate of such systems that is much larger than that of binary pulsars with a massive white dwarf companion, the natural des-cendants of systems like HD 49798 if the pulsar in this system were a neutron star.