S-duality in N=4 supersymmetric gauge theories
  The Goddard, Nuyts and Olive conjecture for electric-magnetic duality in Yang-Mills theory with an arbitrary gauge group G is extended by including a non-vanishing vacuum angle $\theta$. This extended S-duality conjecture includes the case when the unbroken gauge group is non-abelian and a definite prediction for the spectrum of dyons results. 
N. Dorey, C. Fraser, T.J. Hollowood, M.A.C. Kneipp