Gravitational Couplings on D-brane Revisited
  Gravitational couplings in bulk space-time include those terms which are fixed by scattering amplitude of strings and ambiguous terms that are coming from the field redefinitions. These field redefinitions can be fixed in the bulk by ghost-free condition. In this paper we have revised the effective gravitational couplings on D-branes by including the field redefinitions. We find the gravitational effective action up to $\alpha'^2$-order. 
Ahmad Ghodsi and Ghadir Jafari
hep-th