  We consider a modified version of four-dimensional electrodynamics, which has
a photonic Chern-Simons-like term with spacelike background vector in the
action. Light propagation in curved spacetime backgrounds is discussed using
the geometrical-optics approximation. The corresponding light path is modified,
which allows for new effects. In a Schwarzschild background, for example, there
now exist stable bounded orbits of light rays and the two polarization modes of
light rays in unbounded orbits can have different gravitational redshifts. An
Erratum with important corrections has been published, which appears as
Appendix E in this arXiv version.
