  Using the boundary state formalism, we perform a microscopic string analysis
of the interaction between two D-branes and provide a local interpretation for
the R-R force in the D0-D8 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 than field strengths. The Hilbert space of
such R-R states contains combinations of two vectors that decouple from all
physical amplitudes, even in the presence of boundaries. Identifying these
vectors, we remove the null states and recover duality relations among R-R
potentials. If we specify to the D0-D8 brane system, this mechanism implies
that the R-R 1-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-vanishing R-R
contribution in the interaction for the D0-D8 brane system and those related to
it by duality.
