Abstract
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)
.