Probing F-theory with branes

Last week, A. Sen found an explicit type I string compactification dual to the eight-dimensional F-theory construction with SO(8)4 nonabelian gauge symmetry. He found that the perturbations around the enhanced symmetry point were described by the mathematics of the solution of N = 2, d = 4 SU(2) gauge theory with four flavors, and argued more generally that global symmetry enhancement in N = 2, d = 4 gauge theories corresponded to gauge symmetry enhancement in F-theory. We show that these N = 2, d = 4 gauge theories have a physical interpretation in the 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 quantum 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.