  We study the two-loop F^6 interactions in SO(32) heterotic superstring theory
in D=10. By using the generalized Riemann identity we are able to determine the
single-trace part of the effective action up to a few constants which are
related to certain scattering amplitudes. This two-loop heterotic result is
related by duality to Type I interactions at the tree level. However, it turns
out to be completely different from any sort of non-Abelian generalization of
Born-Infeld theory. We offer an explanation of this discrepancy.
