U-duality of Born-Infeld Theory

String theory compactified on a three-torus possesses an SL(5,Z) U-duality group. We investigate the realisation of this symmetry on the Born-Infeld theory on a three-brane, and discuss a U-duality covariant formulation of the BPS sector of the theory where the rank of the gauge group is treated on an equal footing with the fluxes.