A conserved current for the field perturbations in the Einstein–Yang–Mills-dilaton–axion theory

Using the self-adjoint character of the operators governing the field perturbations in the Einstein–Yang–Mills-dilaton–axion theory, we demonstrate that a covariantly (both gauge covariant and space–time covariant) conserved current associated with the coupled field perturbations arises, by contrast to other approaches, in a natural way. Our results cover, in particular, the bosonic low-energy limit of the string theory. We discuss how the present results can serve as a starting point for future investigations.