  Present knowledge of higher-derivative terms in string effective actions is,
with a few exceptions, restricted to the NS-NS sector, a situation which
prevents the development of a variety of interesting applications for which the
RR terms are relevant. We here provide the formalism as well as efficient
techniques to determine the latter directly from string-amplitude calculations.
As an illustration of these methods, we compute the dependence of the type-IIB
action on the three- and five-form RR field strengths at four-point, genus-one,
order-(alpha')^3 level. We explicitly verify that our results are in accord
with the SL(2,Z) S-duality invariance of type-IIB string theory. Extensions of
our method to other bosonic terms in the type-II effective actions are
discussed as well.
