  We search for $N=2$, $d=4$ theories which can be realized both as heterotic
string compactifications on $K_{3}\times T^{2}$ and as type II string
compactifications on Calabi-Yau threefolds. In such cases, the exact
non-perturbative superpotential of one string theory is given in terms of tree
level computations in the other string theory. In particular we find concrete
examples which provide the stringy realization of the results of Seiberg and
Witten on N=2 Yang-Mills theory, corrected by gravitational/stringy effects. We
also discuss some examples which shed light on how the moduli spaces of
different N=2 heterotic vacua are connected.
