Strong Coupling Singularities and Non-abelian Gauge Symmetries in $N=2$   String Theory
  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. 
A. Klemm and P. Mayr