Strong coupling singularities and enhanced non-abelian gauge symmetries in N = 2 string theory Original Research Article

We study a class of extremal transitions between topological distinct Calabi-Yau manifolds which have an interpretation in terms of the special massless states of a type II string compactification. In those cases where a dual heterotic description exists the exceptional massless states are due to genuine strong (string-) coupling effects. A new feature is the appearance of enhanced non-abelian gauge symmetries in the exact nonperturbative theory.