  Expressions for the general and complete perturbations in terms of Debye
potentials of static charged black holes in string theory, valid for curvature
below the Planck scale, are derived starting from a decoupled set of equations
and using Wald's method of adjoint operators. Our results cover both extremal
and nonextremal black holes and are valid for arbitrary values of the dilaton
coupling parameter. The decoupled set is obtained using the Newman-Penrose
formulation of the Einstein-Maxwell-dilaton theory and involves naturally field
quantities invariant under both ordinary gauge transformations of the
electromagnetic potential perturbations and infinitesimal rotations of the
perturbed tetrad. Furthermore, using the recent pointed out relationship
between adjoint operators and conserved currents, a local continuity law for
the field perturbations in terms of the potentials is also obtained. It is
shown that such continuity equation implies the existence of conserved
quantities and of a covariant symplectic structure on the phase space. Future
extensions of the present results are discussed.
