EVIDENCE FOR F-THEORY

We construct compact examples of D-manifolds for type IIB strings. The construction has a natural interpretation in terms of compactificatio n of a 12-dimensional 'F-theory'. We provide evidence for a more natur al reformulation of type IIB theory in terms of F-theory. Compactifica tion of M-theory on a manifold K which admits elliptic fibration is eq uivalent to compactification of F-theory on K x S-1. A large class of N = 1 theories in 6 dimensions are obtained by compactification of F-t heory on Calabi-Yau threefolds. A class of phenomenologically promisin g compactifications of F-theory is on Spin(7) holonomy manifolds down to 4 dimensions. This may provide a concrete realization of Witten's p roposal for solving the cosmological constant problem in four dimensio ns.