S-Duality and the Spectrum of Magnetic Monopoles in Heterotic String Theory

We discuss the predictions of S-duality for the monopole spectrum of four-dimensional heterotic string theory resulting from toroidal compactification. We discuss in detail the spectrum of "H-monopoles", states that are magnetically charged with respect to the U(1) groups arising from the dimensional reduction of the ten dimensional antisymmetric tensor field. Using an assumption concerning the correct treatment of collective coordinates in string theory we find results which are consistent with S-duality.