  The characteristic function for heat fluctuations in a non equilibrium system
is characterised by a large deviation function whose symmetry gives rise to a
fluctuation theorem. In equilibrium the large deviation function vanishes and
the heat fluctuations are bounded. Here we consider the characteristic function
for heat fluctuations in equilibrium, constituting a sub-leading correction to
the large deviation behaviour. Modelling the system by an oscillator coupled to
an explicit multi-oscillator heat reservoir we evaluate the characteristic
function.
