  We investigate the stability of the hidden sector gaugino condensate in a
SL(2,Z)-invariant supergravity model inspired by the E8*E8 heterotic string,
using the chiral superfield formalism. We calculate the Planck-suppressed
corrections to the ``truncated approximation'' for the condensate value and the
scalar potential. A transition to a phase with zero condensate occurs near
special points in moduli space and at large compactification radius. We discuss
the implications for the T-modulus dependence of supersymmetry-breaking.
