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We report on the results we obtained with XMM-Newton observations of HD49798and BD+37 442, the only two sdO stars for which X-ray emission has beenobserved so far. HD is a single-lined spectroscopic binary with orbital periodof 1.5 days. We could establish that its companion is a massive white dwarfwith M = 1.28 Msun, which makes it a candidate type Ia supernova progenitor; wealso detected a significant X-ray emission during the white-dwarf eclipse,which could be X-ray emission of the sdO star itself. In the case of BD+37 442,a luminous He-rich sdO that up to now was believed to be a single star, wediscovered soft X-ray emission with a periodicity of 19.2 s. This indicatesthat also this hot subdwarf has a compact binary companion, either a whitedwarf or a neutron star, most likely powered by accretion from the wind of thesdO star.

Observations of the unique X-ray emitting subdwarf stars HD49798 and BD+37 442