  We review the X-ray observations of hot subdwarf stars. While no X-ray
emission has been detected yet from binaries containing B-type subdwarfs,
interesting results have been obtained in the case of the two luminous O-type
subdwarfs HD 49798 and BD +37 442. Both of them are members of binary systems
in which the X-ray luminosity is powered by accretion onto a compact object: a
rapidly spinning (13.2 s) and massive (1.28 M_sun) white dwarf in the case of
HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD
+37 442. Their study can shed light on the poorly known processes taking place
during common envelope evolutionary phases and on the properties of wind mass
loss from hot subdwarfs.
