F-THEORY DUALS OF NONPERTURBATIVE HETEROTIC E-8XE-8 VACUA IN 6 DIMENSIONS

We present a systematic way of generating F-theory models dual to non- perturbative vacua (i.e. vacua with extra tensor multiplets) of hetero tic E-8 x E-8 strings compactified on K3, using hypersurfaces in toric varieties, In all cases, the Calabi-Yau is an elliptic fibration over a blow-up of the Hirzebruch surface F-n. We find that in most cases t he fan of the base of the elliptic fibration is visible in the dual po lyhedron of the Calabi-Yau, and that the extra tensor multiplets are r epresented as points corresponding to the blow-ups of the F-n. (C) 199 7 Elsevier Science B.V.