Using the boundary state formalism, we perform a microscopic string an
alysis of the interaction between two D-branes and provide a local int
erpretation for the R-R force in the DO-Dg brane system. To do so, we
construct BRST invariant vertex operators for the massless R-R states
in the asymmetric picture that are proportional to potentials rather t
han field strengths. The Hilbert space of such R-R states contains com
binations of two vectors that decouple from all physical amplitudes, e
ven in the presence of boundaries. Identifying these vectors, we remov
e the null states and recover duality relations among R-R potentials.
If we specify to the D0-D8 brane system, this mechanism implies that t
he R-R I-form state has a non-zero overlap with both the D0-brane and
the D8-brane, thus explaining from a local point of view the non-vanis
hing R-R contribution in the interaction for the D0-D8 brane system an
d those related to it by duality. (C) 1998 Elsevier Science B.V.