Dark Radiation candidates after Planck
  Recent Cosmic Microwave Background (CMB) results from the Planck satellite,
combined with previous CMB data and Hubble constant measurements from the
Hubble Space Telescope, provide a constraint on the effective number of
relativistic degrees of freedom of Neff=3.62^{+0.50}_{-0.48} at 95% CL. These
new measurements provide a unique opportunity to place limits on models
containing relativistic species at the decoupling epoch. Here we review the
bounds or the allowed parameter regions in sterile neutrino models, hadronic
axion models as well as on extended dark sectors with additional light species
based on the latest Planck CMB observations.
