We present a general description of electromagnetic RR interactions be
tween 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-ele
ctric and/or magnetic-magnetic interaction corresponds to the usual RR
even spin structure, the magnetic-electric interaction is described b
y 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 four dimensions. (C) 1998 Elsevier Science B.V.