D-strings on D-manifolds

We study the mechanism for the appearance of massless solitons in type II string compactifications. We find that by combining T-duality with strong/weak duality of type IIB in ten dimensions enhanced gauge symmetries and massless solitonic hypermultiplets encountered in Calabi-Yau compactifications can be studied perturbatively using D-strings (the strong/weak dual to type IIB string) compactified on ''D-manifolds''. In particular the nearly massless solitonic states of the type IIB compactifications correspond to elementary states of D-strings. As examples we consider the D-string description of enhanced gauge symmetries for type IIA string compactification on ALE spaces with A(n) singularities and type IIB on a class of singular Calabi-Yau 3-folds. The class we study includes as a special case the conifold singularity in which case the perturbative spectrum of the D-string includes the expected massless hypermultiplet with degeneracy one.