ENHANCED GAUGE-SYMMETRY IN TYPE-II STRING THEORY

We show how enhanced gauge symmetry in type Il string theory compactif ied on a Calabi-Yau threefold arises from singularities in the geometr y of the target space, When the target space of the type IIA string ac quires a genus g curve C of A(N-1) singularities, we find that an SU(N ) gauge theory with g adjoint hypermultiplets appears at the singulari ty. The new massless states correspond to solitons wrapped about the c ollapsing cycles, and their dynamics is described by a twisted supersy mmetric gauge theory on C x R(4). We reproduce this result from an ana lysis of the S-dual D-manifold, We check that the predictions made by this model about the nature of the Higgs branch, the monodromy of peri od integrals, and the asymptotics of the one-loop topological amplitud e are in agreement with geometrical computations. In one of our exampl es we find that the singularity occurs at strong coupling in the heter otic dual proposed by Kachru and Vafa.