  The physics of kinetic mixing between ordinary and mirror photons is
discussed. An important role is played by four linear combinations we dub the
physical photon, the sterile photon, the physical mirror photon, and the
sterile mirror photon. Because of the mass degeneracy between the two gauge
bosons, quantum coherence effects are important. The physical photon becomes a
certain coherent superposition of the bare ordinary photon and the bare mirror
photon. Similarly, the physical mirror photon is another, but {\it not
orthogonal}, coherent superposition. We discuss the physics of the interaction
between physical mirror photons and ordinary matter. Observational signatures
for some hybrid ordinary/mirror binary astrophysical systems are qualitatively
discussed. We show that a small amount of ordinary matter at the center of a
mirror star may make the mirror star observable. We speculate that the recently
reported halo white dwarfs might actually be mirror halo stars.
