M-theory; Duality; D-branes; F-theory; String theory

We present a relatively simple argument showing that H-dyon states required by S-duality of the heterotic string on T6 are present provided that the BPS dyons required by S-duality of N = 4 supersymmetric Yang-Mills theory are present. We also conjecture and provide evidence that H-dyons at singularities where the non-perturbative gauge symmetry is completely broken behave like BPS dyons.