We construct the low-energy effective field theory for a D3-brane in constant
R-R 2-form potential background as the S-dual theory of a D3-brane in NS-NS
B-field background. Despite the non-Abelian algebra of the noncommutative U(1)
gauge symmetry, the electromagnetic duality transformation can be carried out
to all orders, and the dual Lagrangian is given in a compact form. The gauge
algebra is found to be a mixture of a deformed area-preserving diffeomorphism
and the usual U(1) gauge symmetry.Comment: 29 pages, minor change