  Archival observations from the Very Large Array (VLA) at frequencies between
1.4 GHz and 43 GHz of the 6.6-day O6.5-7+O5.5-6 binary Cyg OB2 #5 over 20 years
are re-examined. The aim is to determine the location and character of its
known variable radio emission. The radio emission consists of a primary
component associated with the binary, and a non-thermal source (NE), $0.8" to
the NE. This work reveals that NE shows no evidence of variation demonstrating
that 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 NE
components. The variable radio emission from the primary has a period of
6.7+/-0.3 years which is described by a simple model of a non-thermal source
orbiting within the stellar wind envelope of the binary. Such a model implies
the presence of a third, unresolved stellar companion (Star C) orbiting the
6.6-day binary with a period of 6.7 years. The non-thermal emission arises from
either a WCR between Star C and the binary system, or possibly from Star C
directly. Examination of radial velocity observations suggests reflex motion of
the 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 a
quadruple system.
