  Several recent papers discuss gravitational corrections to gauge couplings
that depend quadratically on the energy. In the framework of the
background-field approach, these correspond in general to adding to the
effective action terms quadratic in the field strength but with higher-order
space-time derivatives. We observe that such terms can be removed by
appropriate local field redefinitions, and do not contribute to physical
scattering-matrix elements. We illustrate this observation in the context of
open string theory, where the effective action includes, among other terms, the
well-known Born-Infeld form of non-linear electrodynamics. We conclude that the
quadratically energy-dependent gravitational corrections are \emph{not}
physical in the sense of contributing to the running of a physically-measurable
gauge coupling, or of unifying couplings as in string theory.
