  An ensemble of short open strings in equilibrium with the heat bath provided
by the Euclidean worldvolume of a stack of Dbranes undergoes a thermal phase
transition to a long string phase. The transition temperature is just below the
string scale. We point out that this phenomenon provides a simple mechanism
within open and closed string theories for altering the strong-electro-weak
coupling unification scale relative to the fundamental closed string mass scale
in spacetimes with external electromagnetic background.
