We report on the results we obtained with XMM–Newton observations of HD 49798 and BD +37 ^ { \circ } 442 , the only two sdO stars for which X–ray emission has been observed so far .
HD 49798 is a single-lined spectroscopic binary with orbital period of 1.5 days .
We could establish that its companion is a massive white dwarf with M = 1.28 M _ { \odot } , which makes it a candidate type Ia supernova progenitor ; we also 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 ^ { \circ } 442 , a luminous He–rich sdO that up to now was believed to be a single star , we discovered soft X–ray emission with a periodicity of 19.2 s. This indicates that also this hot subdwarf has a compact binary companion , either a white dwarf or a neutron star , most likely powered by accretion from the wind of the sdO star .