  We consider the general case of N=2 dual pairs of type IIA/heterotic string
theories in four dimensions. We show that if the type IIA string in this pair
can be viewed as having been compactified on a Calabi-Yau manifold in the usual
way then this manifold must be of the form of a K3 fibration. We also see how
the bound on the rank of the gauge group of the perturbative heterotic string
has a natural interpretation on the type IIA side.
