  In this paper we derive a class of contributions to all orders in $\alpha'$
to the effective action of D-brane-anti D-brane systems in Type II String
Theory, considering an amplitude involving a closed-string Ramond-Ramond state
and four open-string states: two tachyons, a scalar and a gauge boson. This
type of amplitude arises in both Type IIA and Type IIB strings, and reveals a
number of new effective couplings. Furthermore, we derive a series
representation for the result that goes beyond a factorized limit that was
recently studied, and which is expected to apply to more general six-point
amplitudes of superstrings, including external fermions.
