New Black Hole, String and Membrane Solutions of the Four-Dimensional Heterotic String

We present solutions of the low-energy four-dimensional heterotic string corresponding to $p$-branes with $p=0,1,2$, which are characterized by a mass per unit $p$-volume, ${\cal M}_{p+1}$, and topological ``magnetic'' charge, $g_{p+1}$. In the extremal limit, $\sqrt{2} \kappa {\cal M}_{p+1} = g_{p+1}$, they reduce to the recently discovered non-singular supersymmetric monopole, string and domain wall solutions. A novel feature is that the solutions involve both the dilaton and the modulus fields. In particular, 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 $\alpha=1$. This means that electric/magnetic duality in $D=4$ may be seen as a consequence of string/fivebrane duality in $D=10$.