X rays from Wolf-Rayet binaries

It is argued that close binary systems should be fairly intense X-ray sources if one or both components are young massive stars with strong outflow of matter (Wolf-Rayet stars or OB supergiants). Calculations are performed which indicate that the X-radiation from such systems is caused by gas heating behind a shock front formed during the collision of gas flowing from one component with the surface of the second star, its magnetosphere, or with gas flowing from it. The most likely candidates for X-ray sources among Wolf-Rayet binaries are shown to be Gamma-2 Velorum and V444 Cygni.