  We study quantum effects in five dimensions in heterotic superstring theory
compactified on K_3 x S_1 and analyze the conjecture that its dual effective
theory is eleven-dimensional supergravity compactified on a Calabi-Yau
threefold. This theory is also equivalent to type II superstring theory
compactified on the same Calabi-Yau manifold, in an appropriate large volume
limit. In this limit the conifold singularity disappears and is replaced by a
singularity associated to enhanced gauge symmetries, as naively expected from
the heterotic description. Furthermore, we exhibit the existence of additional
massless states which appear in the strong coupling regime of the heterotic
theory and are related to a different type of singular points on Calabi-Yau
threefolds.
