Duality between the webs of heterotic and type II vacua

We discuss how transitions in the space of heterotic K3 x T-2 compactifications are mapped by duality into transitions in the space of type II compactifications on Calabi-Yau manifolds. We observe that perturbative symmetry restoration, as well as non-perturbative processes such as changes in the number of tensor multiplets, have at least in many cases a simple description in terms of the reflexive polyhedra of the Calabi-Yau manifolds. Our results suggest that to many, perhaps all, four-dimensional N = 2 heterotic vacua there are corresponding type II vacua. (C) 1998 Published by Elsevier Science B.V.