  We find that the gas of IIA strings undergoes a phase transition into a gas
of IIB strings at the self-dual temperature. A gas of free heterotic strings
undergoes a Kosterlitz-Thouless duality transition with positive free energy
and positive specific heat but vanishing internal energy at criticality. We
examine the consequences of requiring a tachyon-free thermal string spectrum.
We show that in the absence of Ramond-Ramond fluxes the IIA and IIB string
ensembles are thermodynamically ill-defined. The 10D heterotic superstrings
have nonabelian gauge fields and in the presence of a temperature dependent
Wilson line background are found to share a stable and tachyon-free ground
state at all temperatures starting from zero with gauge group SO(16)xSO(16).
The internal energy of the heterotic string is a monotonically increasing
function of temperature with a stable and supersymmetric zero temperature
limit. Our results point to the necessity of gauge fields in a viable weakly
coupled superstring theory. Note Added (Sep 2005).
