Using the compatibility of the DBI and the Chern-Simons actions with the T-duality transformations, the curvature corrections to these actions have been recently extended to include the quadratic B-field couplings at order O(α′2). In this paper, we use the compatibility of the couplings on D_3-brane with the S-duality to find the nonlinear RR couplings at order O(α′2). We confirm the quadratic RR couplings in the DBI part with the disk-level scattering amplitude. Using the modular functions that appear in the curvature corrections, we then write the results in $SL(2,Z)$ invariant form.

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