F-THEORY AT CONSTANT COUPLING

The subspace of the moduli space of F-theory on K3 over which the coup ling remains constant develops new branches at special values of this coupling. These values correspond to fixed points under the SL(2, Z) d uality group of the type IIB string. The branches contain points where K3 degenerates to orbifolds of the four-torus by Z(3), Z(4) and Z(6). A singularity analysis shows that exceptional group symmetries appear on these branches, including pure E(8) x E(8), although SO(32) cannot be realised in this way. The orbifold points can be mapped to a kind of non-perturbative generalization of a IIB orientifold, and to M-theo ry orbifolds with non-trivial action on 2-brane wrapping modes.