  We show that the duality phase transition in the unoriented type I theory of
open and closed strings is_first order_. The order parameter is the
semiclassical approximation to the heavy quark-antiquark potential at finite
temperature, extracted from the covariant off-shell string amplitude with
Wilson loop boundaries wrapped around the Euclidean time direction. Remarkably,
precise calculations can be carried out on either side of the phase boundary at
the string scale T_C = 1/2\pi \alpha^{'1/2} by utilizing the T-dual, type IB
and type I', descriptions of the short string gas of massless gluon radiation.
We will calculate the change in the duality transition temperature in the
presence of an electromagnetic background field.
