  An accurate determination of the mass-loss rate of the progenitor stars to
core-collapse supernovae is often limited by uncertainties pertaining to
various model assumptions. It is shown that under conditions when the
temperature of the circumstellar medium is set by heating due to free-free
absorption, observations of the accompanying free-free optical depth allow a
direct determination of the mass-loss rate from observed quantities in a rather
model independent way. The temperature is determined self-consistently, which
results in a characteristic time dependence of the free-free optical depth.
This can be used to distinguish free-free heating from other heating
mechanisms. Since the importance of free-free heating is quite model dependent,
this also makes possible several consistency checks of the deduced mass-loss
rate. It is argued that the free-free absorption observed in SN 1993J is
consistent with heating from free-free absorption. The deduced mass-loss rate
of the progenitor star is, approximately, 10^(-5) solar masses per year for a
wind velocity of 10 km/s.
