Comments on T-Dualities of Ramond-Ramond Potentials

The type IIA/IIB effective actions compactified on Td are known to be invariant under the T-duality group SO(d, d; Z), although the invariance of the R-R sector cannot be seen so directly.Inspired by a work of Brace, Morariu and Zumino, we introduce new potentials, which are mixtures of R-R potentials and the NS-NS 2-form, in order to make the invariant structure of R-R sector more transparent. We give a simple proof that if these new potentials transform as a Majorana-Weyl spinor of SO(d, d; Z), the effective actions are indeed invariant under the T-duality group.The argument is made in such a way that it can apply to Kaluza-Klein forms of arbitrary degree. We also demonstrate that these new fields simplify all the expressions, including the Chern-Simons term.