STRING SOLITONS

We review the status of solitons in superstring theory, with a view to understanding the strong coupling regime. These solitonic solutions are non-singular field configurations which solve the empty-space low-energy field equations (generalized, whenever possible, to all in alpha'), carry a non-vanishing topological ''magnetic'' charge and are stabilized by a topological conservation law. They are compared and contrasted with the elementary solutions which are singular solutions of the field equations with a sigma-model source term and carry a non-vanishing Noether ''electric'' charge. In both cases, the solutions of most interest are those which preserve half the space-time supersymmetries and saturate a Bogolol'nyi bound. They typically arise as the extreme mass = charge limit of more general two-parameters solutions with event horizons. We also describe the theory dual to the fundamental string for which the roles of elementary and soliton solutions are interchanged. In 10 space-time dimensions, this dual theory is a superfivebrane and this gives rise to a string\fivebrane duality conjecture according to which the fivebrane may be regarded as fundamental in its own right, with the strongly coupled string corresponding to the weakly coupled fivebrane and vice versa. After compactification to the four space-time dimensions, the fivebrane appears as a magnetic monopole or a dual string according as it wraps around five or four of the compactified dimensions. This gives rise to a four-dimensional string/string duality conjecture which subsumes a Montonen-Olive type duality in that the magnetic monopoles of the fundamental string correspond to the electric winding states of the dual string. This leads to a duality of dualities whereby under string/string duality the strong/weak coupling S-duality trades places with the minimum/maximum length T-duality. Since these magnetic monopoles are extreme black holes, a prediction of S-duality is that the corresponding electric massive states of the fundamental string are also extreme black holes. This is indeed the case.