In heterotic-Type IIA duality, one studies Calabi-Yau threefolds fibred by lattice polarized K3 surfaces on the type IIA side. For a given choice of lattice polarization, there are in general several inequivalent fibrations without reducible fibres. We examine singular transitions between such Calabi-Yau threefolds and examine the reducible K3 fibres which occur at the transition point. We identify such degenerate K3 fibres as solitons generalizing the NS5-brane in heterotic string theory. The theory of degenerations of K3 surfaces can then be used to find which solitons can be present on the heterotic side.

Coauthor: Taizan Watari (IPMU, University of Tokyo)