On a supersymmetric completion of the $R^4$ term in type IIB   supergravity
  We examine the question of the supersymmetric completion of the $R^4$ term in type IIB supergravity by using superfield methods. We show that while there is an obstruction to constructing the full action, a subset of the terms in the action can be consistently analyzed independent of the other terms, and these can be obtained from the superfield. We find the complete type IIB action involving the curvature and five-form field strengths. 
Arvind Rajaraman