  We study moduli stabilization by thermal effects in the cosmological context.
The implementation of finite temperature, which spontaneously breaks
supersymmetry, induces an effective potential at one loop level. At the points
where extra massless states appear in the string spectrum, the potential
develops local minima whose depth depends on the temperature. Moduli attracted
to these points acquire dynamical masses which decrease with cosmological
evolution. This makes the coherent scalar oscillations dilute before
nucleosynthesis, and the cosmological moduli problem is avoided. In particular,
we study the effective potential induced by a maximally supersymmetric
heterotic string gas for spacetime dimension D>=4, and a gas of type II strings
compactified on Calabi-Yau three-folds (D=4). In the former case, the local
minima of the potential arise at enhanced gauge symmetry points, which can
stabilize all moduli but the dilaton. In the latter case, the local minima are
reached at the loci where 2-cycles or 3-cycles in the Calabi-Yau space shrink
to zero size, accompanied with either conifold transitions or non Abelian gauge
symmetries. This stabilizes the type II moduli which characterize the
deformation of these shrinking cycles. Moduli stabilization in the dual string
models is also investigated by heterotic/type I S-dualities and type
II/heterotic dualities.
