  In heterotic string theory compactified to four dimensions with N=2
supersymmetry, string-loop corrections to the universal sector of the
low-energy effective action are studied. Within the framework of N=2
supersymmetric formulation of the theory, in the first order in string coupling
constant, we solve the system of the loop-corrected Maxwell and Killing spinor
equations. Taking as the in-put the tree-level dyonic black hole solution, we
calculate string-loop corrections to the string tree-level metric and moduli of
dyonic black hole.
