  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 $SL(2, Z)$.
