A spectroscopic study of the non-thermal radio emitter Cyg OB2#8A: Discovery of a new binary system

peer reviewedWe present the results of a spectroscopic campaign revealing that the non-thermal radio emitter Cyg OB2 #8A is an O6+O5.5 binary system. We propose the very first orbital solution indicating a period of about 21.9 days. The system appears to be eccentric (0.24+/-0.04) and is likely seen under a rather low inclination angle. The mass ratio of the components is close to unity. The impact of the binarity of this star in the framework of our understanding of non-thermal radio emission from early-type stars is briefly discussed