Orbifolds and solitons

We propose a conformal field theory description of a solitonic heterotic string in type IIA superstring theory compactified on K3, generalizing previous work by J. Harvey, A. Strominger and A. Sen. In ten dimensions the construction gives a fivebrane which is related to the fundamental type II string by electric-magnetic duality, and to the Dirichlet fivebrane of type IIB string theory by SL(2, Z).