  We present a general description of electromagnetic RR interactions between
pairs of magnetically dual D-branes, focusing on the interaction of a
magnetically charged brane with an electrically charged one. In the boundary
state formalism, it turns out that while the electric-electric and/or
magnetic-magnetic interaction corresponds to the usual RR even spin structure,
the magnetic-electric interaction is described by the RR odd spin structure. As
representative of the generic case of a dual pair of p and 6-p branes, we
discuss in detail the case of the self-dual 3-brane wrapped on a T_6/Z_3, which
looks like an extremal dyonic black hole in 4 dimensions.
