SOLITONIC STRINGS AND BPS SATURATED DYONIC BLACK-HOLES

We consider a six-dimensional solitonic string solution described by a conformal chiral null model with nontrivial N=4 superconformal transv erse part. It can be interpreted as a five-dimensional dyonic solitoni c string wound around a compact fifth dimension. The conformal model i s regular with the short-distance (''throat'') region equivalent to a Wess-Zumino-Witten (WZW) theory. At distances larger than the compacti fication scale the solitonic string reduces to a dyonic static spheric ally symmetric black hole of toroidally compactified heterotic string. The new four-dimensional solution is parametrized by five charges, sa turates the Bogomol'nyi bound, and has a nontrivial dilaton-axion fiel d and moduli fields of a two-torus. When acted on by combined T- and S -duality transformations it serves as a generating solution for all th e static spherically symmetric Bogomol'nyi-Prasad-Sommerfield saturate d configurations of the low-energy heterotic string theory compactifie d on a six-torus. Solutions with regular horizons have the global spac e-time structure of extreme Reissner-Nordstrom black holes with the no n-zero thermodynamic entropy which depends only on conserved (quantize d) charge vectors. The independence of the thermodynamic entropy on mo duli and axion-dilaton couplings strongly suggests that it should have a microscopic interpretation as counting degeneracy of underlying str ing configurations. This interpretation is supported by arguments base d on the corresponding six-dimensional conformal field theory. The exp ression for the level of the WZW theory describing the throat region i mplies a renormalization of the string tension by a product of magneti c charges, thus relating the entropy and the number of oscillations of the solitonic string in compact directions.