D-Brane Dynamics in Constant Ramond-Ramond Potentials and Noncommutative Geometry

We study the physics of D-branes in the presence of constant Ramond-Ramond potentials. We first develop a general formalism to analyze open strings in gauge trivial closed string backgrounds, and then apply it both to the RNS string and within Berkovits' covariant formalism, where the results have the most natural interpretation. The most remarkable finding is that, in the presence of a Dp-brane, a small parallel NS-NS B-field and R-R C^(p-1)-field can be gauged away to produce a U(1) field strength on the brane given by F = 1/2 (B - *C^(p-1)), where * is Hodge duality along the brane world-volume. This equation differs from the usually assumed result of F=B. Note, however, that the usual result is, in the D3-brane case, not compatible with S-duality. On the other hand our result, which is derived automatically given the general formalism, offers a non-trivial perturbative check of S-duality