MIRROR SYMMETRY AND THE TYPE-II STRING

If X and Y are a mirror pair of Calabi-Yau threefolds, mirror symmetry should extend to an isomorphism between the type IIA string theory co mpactified on X and the type IIB string theory compactified on Y, with all nonperturbative effects included. We study the implications which this proposal has for the structure of the semiclassical moduli space s of the compactified type II theories. For the type IIB theory, the f orm taken by discrete shifts in the Ramond-Ramond scalars exhibits an unexpected dependence on the B-field.