The discovery of 13s X-ray pulsations from the hydrogen depleted   subdwarf O6 star binary HD49798
  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. 
G.L. Israel, L. Stella, L. Angelini, N.E. White, T.R. Kallman, P.   Giommi and A. Treves