  Various extensions of the Standard Model predict the existence of hidden
photons kinetically mixing with the ordinary photon. This mixing leads to
oscillations between photons and hidden photons, analogous to the observed
oscillations between different neutrino flavors. In this context, we derive new
bounds on the photon-hidden photon mixing parameters using the high precision
cosmic microwave background spectral data collected by the Far Infrared
Absolute Spectrophotometer instrument on board of the Cosmic Background
Explorer. Requiring the distortions of the CMB induced by the photon-hidden
photon mixing to be smaller than experimental upper limits, this leads to a
bound on the mixing angle < 10^{-7}-10^{-5} for hidden photon masses between
10^{-14} eV and 10^{-7} eV. This low-mass and low-mixing region of the hidden
photon parameter space was previously unconstrained.
