O-type stars are among the most massive and hottest known, pounding
their surroundings with intense ultraviolet light and powerful outflows
called stellar winds. NASA's Swift and ESA's XMM-Newton X-ray
observatories took part in a 2011 campaign to monitor the interaction of
two O stars bound together in the same binary system: Cygnus OB2 #9.

Credit: NASA's Goddard Space Flight Center

http://www.nasa.gov/mission_pages/swift/bursts/binary-clash.html