EXACT MONODROMY GROUP OF N=2 HETEROTIC SUPERSTRING

We describe an N = 2 heterotic superstring model of rank 3 which is du al to the type II string compactified on a Calabi-Yau manifold with Be tti numbers b(1,1) = 2 and b(1,2) = 86. We show that the exact duality symmetry found from the type II realization contains the perturbative duality group of the heterotic model, as well as the exact quantum mo nodromies of the rigid SU(2) super-Yang-Mills theory. Moreover, it con tains a non-perturbative monodromy which is stringy in origin and corr esponds roughly to an exchange of the string coupling with the compact ification radius.