Solitonic strings and BPS saturated dyonic black holes.

We consider a six-dimensional solitonic string solution described by a conformal chiral null model with non-trivial N = 4 superconformal transverse part. It can be interpreted as a five-dimensional dyonic solitonic string wound around a compact fifth dimension. The conformal model is regular with the short-distance (‘throat’) region equivalent to a WZW theory. At distances larger than the compactification scale this solitonic string reduces to a static dyonic spherically symmetric black hole of toroidally compactified heterotic string. The new four-dimensional solution is parametrised by five charges, saturates the Bogomol’nyi bound and has nontrivial dilaton-axion field and moduli fields of two-torus. When acted by combined Tand S-duality transformations it serves as a generating solution for all static spherically symmetric BPSsaturated configurations of low-energy heterotic string theory compactified on six-torus. Solutions with regular horizons have global space-time structure of extreme Reissner-Nordström black holes with non-zero thermodynamic entropy which depends only on conserved (quantised) charge vectors The independence of thermodynamic entropy on moduli and axion-dilaton couplings strongly suggests that it should have a microscopic interpretation as counting degeneracy of underlying string configurations. This interpretation is supported by arguments based on corresponding six-dimensional conformal field theory. The expression for the level of the WZW theory describing the throat region implies a renormalisation of string tension by a product of magnetic charges, thus relating the entropy and the number of oscillations of solitonic string in compact directions. On sabbatic leave from the University of Pennsylvania. e-mail: [email protected] On leave from Lebedev Institute, Moscow. e-mail: [email protected]