  Based on the assumption that the target space duality ($T\to 1/T$) is
preserved even nonperturbatively, the properties of static string vacua are
studied. A discussion of the effective four-dimensional supergravity action
based on target-space modular symmetry $SL(2,{\bf Z})$ is presented. The
nonperturbative superpotential removes the vacuum degeneracy with respect to
the compactification modulus ($T$) generically breaks supersymmetry with
negative cosmological constant. Charged matter fields get negative $(mass)^2$
signalling an additional instability of string vacuum and the blowing up of
orbifold singularities. In addition for a class of modularly invariant
potentials topologically stable stringy domain walls of nontrivial compaction
modulus field configuration are found. They are supersymmetric solutions, thus
saturating the Bogomolnyi bound. Their physical implications are discussed.
