  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 CN=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.
