  For typical models of binary statistics, 50-70% of core-collapse supernova
(ccSN) progenitors are members of a stellar binary at the time of the
explosion. Independent of any consequences of mass transfer, this has
observational consequences that can be used to study the binary properties of
massive stars. In particular, the secondary companion to the progenitor of a
Type Ib/c SN is frequently (~50%) the more optically luminous star since the
high effective temperatures of the stripped progenitors make it relatively easy
for a lower luminosity, cooler secondary to emit more optical light.
Secondaries to the lower mass progenitors of Type II SN will frequently produce
excess blue emission relative to the spectral energy distribution of the red
primary. Available data constrain the models weakly. Any detected secondaries
also provide an independent lower bound on the progenitor mass and, for
historical SN, show that it was not a Type Ia event. Bright ccSN secondaries
have an unambiguous, post-explosion observational signature - strong,
blue-shifted, relatively broad absorption lines created by the developing
supernova remnant. These can be used to locate historical SN with bright
secondaries, confirm that a source is a secondary, and, potentially, measure
abundances of ccSN ejecta. Luminous, hot secondaries will reionize the SNR on
time scales of 100-1000 yrs that are faster than reionization by the reverse
shock, creating peculiar HII regions due to the high metallicity and velocities
of the ejecta.
