Discovery of a Compact Companion to the Hot Subdwarf Star BD +37° 442

We report the results of the first X-ray observation of the luminous and helium-rich O-type subdwarf BD +37° 442 carried out with the XMM-Newton satellite in 2011 August. X-ray emission is detected with a flux of about 3× 10^-14 erg cm^-2 s^-1 (0.2-1 keV) and a very soft spectrum, well fit by the sum of a blackbody with temperature kT _BB = 45⁺¹¹ _{- 9} eV, and a power law with a poorly constrained photon index. Significant pulsations with a period of 19.2 s are detected, indicating that the X-ray emission originates in a white dwarf or neutron star companion, most likely powered by accretion from the wind of BD +37° 442.