  We describe the radio evolution of SN 2001ig in NGC 7424, from 700 days of
multi-frequency monitoring with the Australia Telescope Compact Array (ATCA)
and the Very Large Array (VLA). We find that deviations of the radio light
curves at each frequency from the standard "minishell" model are consistent
with density modulations in the circumstellar medium (CSM), which seem to recur
with a period near 150 days. One possibility is that these are due to enhanced
mass-loss from thermal pulses in an AGB star progenitor. A more likely scenario
however is that the progenitor was a Wolf-Rayet star, whose stellar wind
collided with that from a massive hot companion on an eccentric 100 day orbit,
leading to a regular build-up of CSM material on the required time and spatial
scales. Recent observations of "dusty pinwheels" in Wolf-Rayet binary systems
lend credibility to this model. Since such binary systems are also thought to
provide the necessary conditions for envelope-stripping which would cause the
Wolf-Rayet star to appear as a Type Ib/c supernova event rather than a Type II,
these radio observations of SN 2001ig may provide the key to linking Type Ib/c
SNe to Type IIb events, and even to some types of Gamma-Ray Bursts.
