Non-Perturbative Properties of Heterotic String Vacua Compactified on   ${K3\times T^2}$
  Using the heterotic--type II duality of $N=2$ string vacua in four space-time dimensions we study non-perturbative couplings of toroidally compactified six-dimensional heterotic vacua. In particular, the heterotic--heterotic $S$-duality and the Coulomb branch of tensor multiplets observed in six dimensions are studied from a four-dimensional point of view. We explicitly compute the couplings of the vector multiplets of several type II vacua and investigate the implications for their heterotic duals. 
J. Louis, J. Sonnenschein, S. Theisen, S. Yankielowicz