ON THE UBIQUITY OF K3 FIBRATIONS IN STRING DUALITY

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 stri ng 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 o f a K3 fibration. We also see how the bound on the rank of the gauge g roup of the perturbative heterotic string has a natural interpretation on the type IIA side.