S-duality via outer-automorphism twists

Compactification of 6D theory of type G on a punctured Riemann surface has been effectively used to understand S-dualities of 4D theories. We can further introduce branch cuts on the Riemann surface across which the worldvolume fields are transformed by the discrete symmetries associated with those of the Dynkin diagram of type G. This allows us to generate more S-dualities, and in particular to reproduce a couple of S-dual pairs found previously by Argyres and Wittig.