  The one-loop vacuum energy is explicitly computed for a class of perturbative
string vacua where supersymmetry is spontaneously broken by a T-duality
invariant asymmetric Scherk-Schwarz deformation. The low-lying spectrum is
tachyon-free for any value of the compactification radii and thus no
Hagedorn-like phase-transition takes place. Indeed, the induced effective
potential is free of divergence, and has a global anti de Sitter minimum where
geometric moduli are naturally stabilised.
