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 \pm 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 .