  We study the consequences of the constant-temperature behaviour of nuclei in
the superfluid regime for the exchange of excitation energy between two nuclei
in thermal contact. This situation is realized at the scission configuration of
fission at moderate excitation energies. It is shown that all available
excitation energy is transferred to the colder fragment. This effect explains
why an increase of excitation energy is translated into an increase of the
number of emitted neutrons for the heavy fission fragments only. This
observation remained unexplained up to now.
