Black hole solutions in string theory

We present two-parameter solutions of the low-energy four-dimensional heterotic string which in the extremal limit reduce to supersymmetric monopole, string and domain wall solutions. The effective scalar coupling to the Maxwell field, e^{-\alpha \phi} F_{\mu\nu} F^{\mu\nu}, gives rise to a new string black hole with \alpha = \sqrt{3}, in contrast to the pure dilaton black hole solution which has \alpha=1. Implications of string/fivebrane duality in D=10 to four-dimensional dualities are discussed.