NEW BRANCHES OF STRING COMPACTIFICATIONS AND THEIR F-THEORY DUALS

We study heterotic E-8 x E-8 models that are dual to compactifications of F-theory and type IIA string on certain classes of elliptically fi bered Calabi-Yau manifolds. Different choices for the specific torus i n the fibration have heterotic duals that are most easily understood i n terms of E-8 x E-8 models with gauge backgrounds of type H x U(1)(8- d), where H is a non-Abelian factor. The case with d = 8 corresponds t o the well-known E-8 x E-8 compactifications with nonAbelian instanton backgrounds (k(1), k(2)) whose F-theory duals are built through compa ctifications on fibrations of the torus P-2((1,2,3)) over F-n. The new cases with d < 8 correspond to other choices for the elliptic fiber o ver the same base and yield unbroken U(1)'s, some of which are anomalo us and acquire a mass by swallowing zero-modes of the antisymmetric B- MN field. We also study transitions to models with no tensor multiplet s in D = 6 and find evidence of E-d instanton dynamics. We also consid er the possibility of conifold transitions among spaces with different realization of the elliptic fiber. (C) 1997 Elsevier Science B.V.