S-DUALITY AND THE DYON SPECTRUM IN N=2 SUPER YANG-MILLS THEORY

We study the dyon spectrum in N = 2 super Yang-Mills theory with gauge group SU(2) coupled to N-f matter multiplets in the fundamental repre sentation. For magnetic charge one and two we determine the spectrum e xplicitly and show that it is in agreement with the duality prediction s of Seiberg and Witten. We briefly discuss the extension to higher ch arge monopoles for the self-dual N-f = 4 case and argue that the conje ctured spectrum of dyons predicts the existence of certain harmonic sp inors on the moduli space of higher charge monopoles.