F-theory from Dirichlet 3-branes

Starting from the type IIB Dirichlet 3-brane action, we obtain a Nambu-Goto action. It is interpreted as the world volume action of a fundamental 3-brane, and its target space theory as F-theory. The target space is twelve dimensional, with signature (11, 1). It is an elliptic fibration over a ten dimensional base space. The SL(2, Z) symmetry of type IIB string has now an explicit geometric interpretation. Also, one gets a glimpse of the conjectured self-duality of M-theory.