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We discovered strong 13s X-ray pulsations in the public Rosat PSPC lightcurve of HD49798, a 1.55d single-component spectroscopic binary containing ahydrogen depleted subdwarf O6 star. We find no evidence for period changesduring the about 4hr Rosat pointing. The source X-ray spectrum is extremelysoft, 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. Ourresults imply that the unseen companion is an accreting degenerate star, awhite dwarf or, more likely, a neutron star. In either case HD49798 correspondsto a previously unobserved evolutionary stage of a massive binary system, aftercommon envelope and spiral-in.Comment: 10 pages plus 2 postscript figures. AASTeX. Accepted for publication  in the Astrophysical Journal Letter

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