AN N=2 DUAL PAIR AND A PHASE-TRANSITION

We carefully analyze the N = 2 dual pair of string theories in four di mensions introduced by Ferrara, Harvey, Strominger and Vafa. The analy sis shows that a second discrete degree of freedom must be switched on in addition to the known ''Wilson line'' to achieve a non-perturbativ ely consistent theory. We also identify the phase transition this mode l undergoes into another dual pair via a process analogous to a conifo ld transition. This provides the first known example of a phase transi tion which is understood from both the type II and the heterotic strin g picture.