  Doubled $\alpha'$-geometry is the simplest higher-derivative gravitational
theory with exact global duality symmetry. We use the double metric formulation
of this theory to compute on-shell three-point functions to all orders in
$\alpha'$. A simple pattern emerges when comparing with the analogous bosonic
and heterotic three-point functions. As in these theories, the amplitudes
factorize. The theory has no Gauss-Bonnet term, but contains a Riemann-cubed
interaction to second order in $\alpha'$.
