Stringy domain walls and target-space modular invariance

Abstract
Based on the target-space modular invariance of the nonperturbative superpotential of the four-dimensional N=1 supersymmetric string vacua, we find topologically stable, stringy domain walls of nontrivial compactification modulus field configurations. They are supersymmetric solutions, thus saturating the Bogomolnyi bound. Their physical implications are discussed.