  We discuss some striking properties of photons propagating in a cold axion
condensate oscillating coherently in time with a frequency $1/m_a$. Three
effects are discussed in this contribution: (a) due to the time dependence of
the background, photons moving in the cold axion background have no definite
energies and some momenta are not accessible to them. (b) we investigate the
combined influence of a magnetic field and the cold axion background and
propose a possible interferometric experiment to detect the latter. (c) if the
axion condensate has a space dependence, the photon refraction index is
modified in the medium, possibly leading to total reflection at the interface
with the ordinary vacuum.
