Thanks to the high sensitivity of the instruments on board the XMM-Newton and
Chandra satellites, it has become possible to explore the properties of the
X-ray emission from hot subdwarfs. The small but growing sample of hot
subdwarfs detected in X-rays includes binary systems, in which the X-rays
result from wind accretion onto a compact companion (white dwarf or neutron
star), as well as isolated sdO stars in which X-rays are probably due to shock
instabilities in the wind. X-ray observations of these low mass stars provide
information which can be useful also for our understanding of the winds of more
luminous and massive early-type stars and can lead to the discovery of
particularly interesting binary systems.Comment: Review paper to be published in Advances in Space Research (special
issue on "X-ray emission from hot stars and their winds"
