Cyg OB2 #9: XMM-Newton and Swift observations
The Cygnus OB2 #9 binary system consists of two nearly equal-mass O stars that follow an eccentric orbit around their common centre of gravity.
Observations with ESA's XMM-Newton and NASA's Swift observatories showed a quadrupling of X-ray flux around the time of periastron (June/July 2011) - when the stars were close together in their orbits. This has provided the first concrete evidence that a wind-wind collision is taking place in this system.
Previous observations with XMM-Newton had hinted at the presence of colliding stellar winds but those observations were carried out when the two stars were far from each other.