  On compactification to six spacetime dimensions, the fundamental heterotic
string admits as a soliton a dual string whose effective worldsheet action
couples to the background fields of the dual formulation of six-dimensional
supergravity. On further toroidal compactification to four spacetime
dimensions, the dual string acquires an $O(2,2;Z)$ target-space duality. This
contains as a subgroup the axion-dilaton $SL(2,Z)$ which corresponds to a
strong/weak coupling duality for the fundamental string. The dual string also
provides a new non-perturbative mechanism for enhancement of the gauge
symmetry.
