  Axion modification of the electrodynamics can be considered as produced by an
irreducible part of the constitutive pseudotensor. In this paper, we study the
modification of wave propagation produced by the second irreducible part called
skewon. We introduce the notions of skewon optic tensor and skewon optic
covector. With these devices we prove that in a pseudo-Riemannian manifold
endowed with an arbitrary skewon at least one solution of the dispersion
relation is spacelike. This means that the skewon generates superluminal wave
motion and is thus ruled out on the basis of SR principles.
