Tree-level R⁴ correction from $O(d,d)$: NS-NS five-point terms

The tree-level string effective action reduced from D to $D-d$ dimensions possesses a continuous $O(d,d)$ symmetry, closely related to T-duality. A necessary condition for a higher derivative correction to preserve this symmetry is that certain $O(d,d)$ violating terms which appear in the dimensional reduction have to cancel out. We use this idea to complete the quartic Riemann correction with all terms involving five NS-NS sector fields. The resulting Lagrangian is considerably simpler than expressions that have previously appeared in the literature.