PROBING F-THEORY WITH BRANES

Last week, A. Sen found an explicit type I string compactification dua l to the eight-dimensional F-theory construction with SO(8)(4) nonabel ian gauge symmetry. He found that the perturbations around the enhance d symmetry point were described by the mathematics of the solution of N = 2, d = Lf SU(2) gauge theory with four flavors, and argued more ge nerally that global symmetry enhancement in N = 2, d = 4 gauge theorie s corresponded to gauge symmetry enhancement in F-theory. We show that these N = 2, d = 4 gauge theories have a physical interpretation in t he theory. They are the world-volume theories of 3-branes parallel to the 7-branes. They can be used to probe the structure of the exact qua ntum F-theory solutions. On the Higgs branch of the moduli space, the objects are equivalent to finite size instantons in the 7-brane gauge theory.