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The Discovery of 13 second X-Ray Pulsations from the Hydrogen-depleted Subdwarf O6 Star Binary HD 49798

We have discovered strong ~13 s X-ray pulsations in the ROSAT Position Sensitive Proportional Counter light curve of HD 49798, a 1.55 day single-component spectroscopic binary containing a hydrogen-depleted subdwarf O6 star. We find no evidence for period changes during the ~4 hr ROSAT pointing. The source X-ray spectrum is extremely soft, with an unabsorbed 0.1-2 keV luminosity of a few times 1032 ergs 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 any case, HD 49798 corresponds to a previously unobserved evolutionary stage of a massive binary system after the common-envelope phase and spiraling in.