  Type II supernova progenitors are expected to emit copious amounts of mass in
a dense stellar wind prior to the explosion. When the progenitor is a member of
a binary, the orbital motion modulates the density of this wind. When the
progenitor explodes, the high-velocity ejecta collides with the modulated wind,
which in turn produces a modulated radio signal. In this work we derive general
analytic relations between the parameters of the radio signal modulations and
binary parameter in the limit of large member mass ratio. We use these
relations to infer the semi major axis of SN1979c and a lower bound for the
mass of the companion. We further constrain the analytic estimates by numerical
simulations using the AMUSE framework. In these calculations we simulate the
progenitor binary system including the wind and the gravitational effect of a
companion star. The simulation output is compared to the observed radio signal
in supernova SN1979C. We find that it must have been a binary with an orbital
period of about 2000 year. If the exploding star evolved from a $\sim 18
M_{\odot}$ zero-age main-sequence at solar metalicity, we derive a companion
mass of $5$ to $12 M_{\odot}$ in an orbit with an eccentricity lower than about
0.8.
