  With the exception of a few well-known and studied systems, the binary
population of red supergiants (RSGs) remains relatively uncharacterized. Famous
systems such as VV Cep, 31 Cyg and zeta Aur contain RSG + B star binaries and
here we explore whether B stars are the main type of companion we expect from
an evolutionary point of view. Using the Geneva evolutionary models we find
that this is indeed the case. However, few such systems are known, and we use
model spectra to determine how easy such binaries would be to detect
observationally. We find that it should be quite difficult to hide a B-type
companion given a reasonable signal-to-noise in the optical / blue portion of
the spectrum. We next examine spectra of Magellanic Cloud RSGs and newly
acquired spectra of Galactic RSGs looking for new systems and refining our
conclusions about what types of stars could be hidden in the spectra. Finally,
we develop a set of photometric criteria that can help select likely binaries
in the future without the overhead of large periodic or spectroscopic surveys.
