HETEROTIC GAUGE STRUCTURE OF TYPE-II K3 FIBRATIONS

We show that certain classes of K3 fibered Calabi-Yau manifolds derive from orbifolds of global products of K3 surfaces and particular types of curves. This observation explains why the gauge groups of the hete rotic duals are determined by the structure of a single K3 surface and provides the dual heterotic picture of conifold transitions between K 3 fibrations. Abstracting our construction from the special case of K3 hypersurfaces to general K3 manifolds with an appropriate automorphis m, we show how to construct Calabi-Yau threefold duals for heterotic t heories with arbitrary perturbative gauge groups. This generalization reveals that the previous limit on the Euler number of Calabi-Yau mani folds is an artifact of the restriction to the framework of hypersurfa ces.