THE DISCOVERY OF 13 SECOND X-RAY PULSATIONS FROM THE HYDROGEN-DEPLETED SUBDWARF-06 STAR BINARY HD-49798

We have discovered strong similar to 13 s X-ray pulsations in the ROSA T Position Sensitive Proportional Counter light curve of HD 49798, a 1 .55 day single-component spectroscopic binary containing a hydrogen-de pleted subdwarf O6 star. We find no evidence for period changes during the similar to 4 hr ROSAT pointing. The source X-ray spectrum is extr emely soft, with an unabsorbed 0.1-2 keV luminosity of a few times 10( 32) ergs s(-1) (distance of 650 pc). A higher luminosity might be hidd en in the EUV. Our results imply that the unseen companion is an accre ting degenerate star, a white dwarf, or, more likely, a neutron star. In any case, HD 49798 corresponds to a previously unobserved evolution ary stage of a massive binary system after the common-envelope phase a nd spiraling in.