We examine the sigma model Riemann curvature corrections to the supergravity action under T-duality transformations. Using the compatibility of the effective action with on-shell linear T-duality and with the S-matrix calculations as guiding principles, we have incorporated in this action the couplings of four $B$-field strengths and the couplings of two Riemann curvatures and two $B$-field strengths at order $\alpha'^3$. Using the S-matrix calculations we have also found new dilaton couplings in the string frame at this order.

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