  We study the physics of D-branes in the presence of constant Ramond-Ramond
potentials. In the string field theory context, 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, both a constant
parallel NS-NS B-field and R-R C^(p-1)-field do not solve the open/closed
equations of motion, and induce the same non-vanishing open string tadpole.
After solving the open string equations in the presence of this tadpole, and
after gauging away the closed string fields, one is left with 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. One observes that this result differs from the
usually assumed result F=B. Technically, this is due to the fact that
supersymmetric and bosonic string world-sheet theories are different. Note,
however, that the usual F+B combination is still the combination which remains
gauge invariant at the sigma-model level. Also, the standard result F=B 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 check of S-duality, to all orders in F, and this leads to a S-dual
invariant Moyal deformation. In an appendix, we solve the source equation
describing the open superstring in a generic NS-NS and R-R closed string
background, within the super Poincare covariant formalism.
