  The progenitors of Type Ia and some core collapse supernovae are thought to
be stars in binary systems, but little observational evidence exists to confirm
the hypothesis. We suggest that the collision of the supernova ejecta with its
companion star should produce detectable emission in the hours and days
following the explosion. The interaction occurs at distances ~10^11-10^13 cm
and shocks the impacting supernova debris, dissipating kinetic energy and
re-heating the gas. Initially, some radiation may escape promptly through the
evacuated region of the shadowcone, producing a bright X-ray (0.1-2 keV) burst
lasting minutes to hours with luminosity L ~ 10^44 ergs/s. Continuing radiative
diffusion from deeper layers of shock heated ejecta produces a longer lasting
optical/UV emission which exceeds the radioactively powered luminosity of the
supernova for the first few days after the explosion. These signatures are
prominent for viewing angles looking down upon the shocked region, or about 10%
of the time. The properties of the emission provide a straightforward measure
of the separation distance between the stars and hence (assuming Roche lobe
overflow) the companion's radius. Current optical and UV data sets likely
already constrain red giant companions. By systematically acquiring early time
data for many supernovae, it should eventually be possible to empirically
determine how the parameters of the progenitor system influence the outcome of
the explosion.
