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Aims: We wish to study the origin of the X-ray emission of three massivestars in the Cyg OB2 association: Cyg OB2 #5, #8A, #12. Methods: To this aim,dedicated X-ray observations from XMM and Swift are used, as well as archivalROSAT and Suzaku data. Results: Our results on Cyg OB2 #8A improve the phasecoverage of the orbit and confirm previous studies: the signature of awind-wind collision is conspicuous. In addition, signatures of a wind-windcollision are also detected in Cyg OB2 #5, but the X-ray emission appears to beassociated with the collision between the inner binary and the tertiarycomponent orbiting it with a 6.7yr period, without a putative collision insidethe binary. The X-ray properties strongly constrain the orbital parameters,notably allowing us to discard some proposed orbital solutions. To improve theknowledge of the orbit, we revisit the light curves and radial velocity of theinner binary, looking for reflex motion induced by the third star. Finally, theX-ray emission of Cyg OB2 #12 is also analyzed. It shows a marked decrease inrecent years, compatible with either a wind-wind collision in a wide binary orthe aftermath of a recent eruption.

Wind collisions in three massive stars of Cyg OB2