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Pulsed X-ray emission in the luminous, helium-rich sdO BD+37 442 has recentlybeen discovered (La Palombara et al., 2012). It was suggested that the sdO starhas a neutron star or white dwarf companion with a spin period of 19.2 s. AfterHD 49798, which has a massive white dwarf companion spinning at 13.2 s in an1.55 day orbit, this is only the second O-type subdwarf from which X-rayemission has been detected. We report preliminary results of our ongoingcampaign to obtain time-resolved high-resolution spectroscopy using the CAFEinstrument at Calar Alto observatory and SARG at the Telescopio NationaleGalileo. Atmospheric parameters were derived via a quantitative NLTE spectralanalysis. The line fits hint at an unusually large projected rotation velocity.Therefore it seemed likely that BD+37 442 is a binary similar to HD 49798 andthat the orbital period is also similar. The level of X-ray emission from BD+37442 could be explained by accretion from the sdO wind by a neutron starorbiting at a period of less than ten days. Hence, we embarked on radialvelocity monitoring in order to derive the binary parameters of the BD+37 442system and obtained 41 spectra spread out over several month in 2012. Unlikefor HD\,49798, no radial velocity variations were found and, hence, there is nodynamical evidence for the existence of a compact companion yet. The origin ofthe pulsed X-ray emission remains as a mystery.

The mysterious sdO X-ray binary BD+37 442