Supersymmetric Yang-Mills theory at order alpha'^3
  We construct the order alpha'^3 terms in the supersymmetric Yang-Mills action in ten dimensions for an arbitrary gauge group. The result can be expressed in terms of the structure constants of the Yang-Mills group, and is therefore independent of abelian factors. The alpha'^3 invariant obtained here is independent of the alpha'^2 invariant, and we argue that additional superinvariants will occur at all odd orders of alpha'. 
A. Collinucci, M. de Roo, M. G. C. Eenink