  We consider closed type II and orientifold backgrounds where supersymmetry is
spontaneously broken by asymmetric geometrical fluxes. We show that these can
be used to describe thermal ensembles with chemical potentials associated to
"gravito-magnetic" fluxes. The thermal free energy is computed at the one-loop
string level, and it is shown to be free of the usual Hagedorn-like
instabilities for a certain choice of the chemical potentials. In the closed
string gravitational sector, as well as in the open string matter sector of the
proposed orientifold construction, the free energy turns out to have
"Temperature duality" symmetry, ${\cal F}(T/T_H)={T^2\over T_H^2} {\cal
F}(T_H/T)$, which requires interchanging the space-time spinor representations
$S\leftrightarrow C$. For small temperatures, $T\to 0$, the anti-spinor $C$
decouples from the spectrum while for large temperatures, $T\to \infty$, the
spinor $S$ decouples. In both limits the free energy vanishes, as we recover a
conventional type II superstring theory. At the self dual point $T=T_H$, the
thermal spectra of $S$ and $C$ are identical. Moreover, there are extra
massless scalars in the adjoint representation of an SO(4) non-abelian gauge
symmetry in the closed-string sector, and open-string massless states charged
simultaneously under both the Chan-Paton and the closed-string SO(4) gauge
group.
