1988 20160701171656.0 10.1051/0004-6361:200809367 DOI ASTROimport-145 van Loo, S. 2008 Non-thermal radio emission from O-type stars. III. Is Cygnus OB2 No. 9 a wind-colliding binary? The star Cyg OB2 No. 9 is a well-known non-thermal radio emitter. Recent theoretical work suggests that all such O-stars should be in a binary or a multiple system. However, there is no spectroscopic evidence of a binary component. Re-analysis of radio observations from the VLA of this system over 25 years has revealed that the non-thermal emission varies with a period of 2.35±0.02 yr. This is interpreted as a strong suggestion of a binary system, with the non-thermal emission arising in a wind-collision region. We derived some preliminary orbital parameters for this putative binary and revised the mass-loss rate of the primary star downward from previous estimates. Appendix A is only available in electronic form at http://www.aanda.org Blomme, R. Dougherty, S. M. Runacres, M. C. Astronomy and Astrophysics 483 2 2008 585-590 http://esoads.eso.org/abs/2008A%26A...483..585V published in REFERD