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Archival observations from the Very Large Array (VLA) at frequencies between1.4 GHz and 43 GHz of the 6.6-day O6.5-7+O5.5-6 binary Cyg OB2 #5 over 20 yearsare re-examined. The aim is to determine the location and character of itsknown variable radio emission. The radio emission consists of a primarycomponent associated with the binary, and a non-thermal source (NE), $0.8" tothe NE. This work reveals that NE shows no evidence of variation demonstratingthat the variable emission arises in the primary component. With NE constant,the radio flux from the primary can now be well determined for the first time,especially in observations that do not resolve both the primary and NEcomponents. The variable radio emission from the primary has a period of6.7+/-0.3 years which is described by a simple model of a non-thermal sourceorbiting within the stellar wind envelope of the binary. Such a model impliesthe presence of a third, unresolved stellar companion (Star C) orbiting the6.6-day binary with a period of 6.7 years. The non-thermal emission arises fromeither a WCR between Star C and the binary system, or possibly from Star Cdirectly. Examination of radial velocity observations suggests reflex motion ofthe binary due to Star C, for which a mass of 23^+22_-14 solar mass is deduced.Together with the star associated with NE, this implies that Cyg OB2 #5 is aquadruple system.

Cyg OB2 #5: When three stars are just not enough