STRINGS AS SOLITONS AND BLACK-HOLES AS STRINGS

Supersymmetric closed string theories contain an infinite tower of BPS -saturated, oscillating, macroscopic strings in the perturbative spect rum. When these theories have dual formulations, this tower of states must exist nonperturbatively as solitons in the dual theories. We pres ent a general class of exact solutions of low-energy supergravity that corresponds to al these states. After dimensional reduction they can be interpreted as supersymmetric black holes with a degeneracy related to the degeneracy of the string states. For example, in four dimensio ns we obtain a point-like solution which is asymptotic to a stationary , rotating, electrically-charged black hole with Regge-bounded angular momentum and with the usual ring-singularity replaced by a string sou rce. This further supports the idea that the entropy of supersymmetric black holes can be understood in terms of counting of string stares. We also discuss some applications of these solutions to string duality .