  Common-envelope events (CEEs), during which two stars temporarily orbit
within a shared envelope, are believed to be vital for the formation of a wide
range of close binaries. For decades, the only evidence that CEEs actually
occur has been indirect, based on the existence of systems that could not be
otherwise explained. Here we propose a direct observational signature of CEE
arising from a physical model where emission from matter ejected in a CEE is
controlled by a recombination front as the matter cools. The natural range of
timescales and energies from this model, as well the expected colors,
light-curve shapes, ejection velocities and event rate, match those of a
recently-recognized class of red transient outbursts.
