  The RS CVn-type binary $\sigma$ Geminorum was observed during a large,
long-duration flare simultaneously with {\it XMM-Newton} and the VLA. The light
curves show a characteristic time dependence that is compatible with the
Neupert effect observed in solar flares: The time derivative of the X-ray light
curve resembles the radio light curve. This observation can be interpreted in
terms of a standard flare scenario in which accelerated coronal electrons reach
the chromosphere where they heat the cool plasma and induce chromospheric
evaporation. Such a scenario can only hold if the amount of energy in the fast
electrons is sufficient to explain the X-ray radiative losses. We present a
plausibility analysis that supports the chromospheric evaporation model.
