NONPERTURBATIVE PROPERTIES OF HETEROTIC STRING VACUA COMPACTIFIED ON K3XT(2)

Using the heterotic-type II duality of N = 2 string vacua in four spac e-time dimensions we study non-perturbative couplings of toroidally co mpactified six-dimensional heterotic vacua, In particular, the heterot ic-heterotic S-duality and the Coulomb branch of tensor multiplets obs erved in six dimensions are studied from a four-dimensional point of v iew. We explicitly compute the couplings of the vector multiplets of s everal type II vacua and investigate the implications for their hetero tic duals.