EXACT RESULTS FOR N=2 COMPACTIFICATIONS OF HETEROTIC STRINGS

We search for N = 2, d = 4 theories which can be realized both as hete rotic string compactifications on K3 x T-2 and as type II string compa ctifications on Calabi-Yau threefolds. In such cases, the exact non-pe rturbative superpotential of one string theory is given in terms of tr ee level computations in the other string theory. In particular we fin d concrete examples which provide the stringy realization of the resul ts of Seiberg and Witten on N = 2 Yang-Mills theory, corrected by grav itational/stringy effects. We also discuss some examples which shed li ght on how the moduli spaces of different N = 2 heterotic vacua are co nnected.