  We study the effective physics of F-theory at order $\alpha'^3$ in derivative
expansion. We show that the ten-dimensional type IIB eight-derivative couplings
involving the graviton and the axio-dilaton naturally descend from pure gravity
in twelve dimensions. Upon compactification on elliptically fibered Calabi-Yau
fourfolds, the non-trivial vacuum profile for the axio-dilaton leads to a new,
genuinely N=1, $\alpha'^3$ correction to the four-dimensional effective action.
