  We discovered strong 13s X-ray pulsations in the public Rosat PSPC light
curve of HD49798, a 1.55d single-component spectroscopic binary containing a
hydrogen depleted subdwarf O6 star. We find no evidence for period changes
during the about 4hr Rosat pointing. The source X-ray spectrum is extremely
soft, with an unabsorbed 0.1-2keV luminosity of a few times 10^32 erg s^-1
(distance of 650pc). A higher luminosity might be hidden in the EUV. Our
results imply that the unseen companion is an accreting degenerate star, a
white dwarf or, more likely, a neutron star. In either case HD49798 corresponds
to a previously unobserved evolutionary stage of a massive binary system, after
common envelope and spiral-in.
