  We study in a systematic and modular invariant way gaugino condensation in
the hidden sector as a potential source of hierarchical supersymmetry breaking
and a non--trivial potential for the dilaton $S$ whose real part corresponds to
the tree level gauge coupling constant (${\rm Re}\ S\sim g_{gut}^{-2}$). For
the case of pure Yang--Mills condensation, we show that no realistic results
(in particular no reasonable values for ${\rm Re}\ S$) can emerge, even if the
hidden gauge group is not simple. However, in the presence of hidden matter
(i.e. the most frequent case) there arises a very interesting class of
scenarios with two or more hidden condensing groups for which the dilaton
dynamically acquires a reasonable value (${\rm Re}\ S\sim 2$) and supersymmetry
is broken at the correct scale ($m_{3/2}\sim 10^3\ GeV$) with no need of
fine--tuning. Actually, good values for ${\rm Re}\ S$ and $m_{3/2}$ are
correlated. We make an exhaustive classification of the working possibilities.
Remarkably, the results are basically independent from the value of
$\delta^{GS}$ (the contributions from the Green--Schwarz mechanism). The radius
of the compactified space also acquires an expectation value, breaking duality
spontaneously.
