Heterotic-Type IIA Duality and Degenerations of K3 Surfaces

Understand

We study the duality between four-dimensional N=2 compactifications of heterotic and type IIA string theories.

• Via adiabatic fibration of the duality in six dimensions, type IIA string theory compactified on a K3-fibred Calabi-Yau threefold has a potential heterotic dual compactification.
• This adiabatic picture fails whenever the K3 fibre degenerates into multiple components over points in the base of the fibration.
• Guided by monodromy, we identify such degenerate K3 fibres as solitons generalizing the NS5-brane in heterotic string theory.