Stringy Domain Walls and Other Stringy Topological Defects

We point out that the moduli sector of the $(2,2)$ string compactification with its nonperturbatively preserved non-compact symmetries is a framework to study global topological defects. Based on the target space modular invariance of the nonperturbative superpotential of the four-dimensional $N=1$ supersymmetric string vacua, topologically stable stringy domain walls are found. Explicit supersymmetric solutions for the modulus field and the metric, which saturate the Bogomol'nyi bound, are presented. They interpolate between {\it non-degenerate} vacua. As a corollary, this defines a new notion of vacuum degeneracy of supersymmetric vacua. Nonsupersymmetric stringy domain walls are discussed as well. The moduli sectors with more than one modulus and the non-compact continous symmetry preserved allow for global monopole-type and texture-type configurations.