We discovered strong \sim 13 s X–ray pulsations in the Rosat PSPC light curve of HD49798 , a 1.55 d single–component spectroscopic binary containing a hydrogen depleted subdwarf O6 star .
We find no evidence for period changes during the \sim 4 hr Rosat pointing .
The source X–ray spectrum is extremely soft , with an unabsorbed 0.1–2 keV luminosity of a few \times 10 ^ { 32 } erg s ^ { -1 } ( distance of 650 pc ) .
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 .