  By explicit calculations of four-field couplings, we observe that the higher
derivative corrections to the DBI action in flat space-time, can be either in a
covariant form or in a T-duality invariant form. The two forms are related by a
non-covariant field redefinition. Using this observation, we then propose a
non-covariant but T-duality invariant action which includes all orders of
massless fields and has two extra derivatives with respect to the DBI action.
