  The description of D-branes as boundary states for type II string theories
(in the covariant formulation) requires particular care in the R-R sector. Also
the vertices for R-R potentials that can couple to D-branes need a careful
handling. As an illustration of this, the example of the D0-D8 system is
reviewed, where a ``microscopic'' description of the interaction via exchange
of R-R potentials becomes possible.
