Effect of neutrino rest mass on ionization equilibrium freeze-out
  We discuss how small neutrino rest masses can increase the expansion rate near the photon decoupling epoch in the early universe, causing an earlier, higher temperature freeze-out for ionization equilibrium compared to the massless neutrino case. This yields a larger free-electron fraction. A larger ratio of the sound horizon to the photon diffusion length follows, implying a smaller inferred Neff. This neutrino-mass/recombination effect depends strongly on the neutrino rest masses. Though below current sensitivity, this effect could be probed by next-generation cosmic microwave background experiments, giving an observational handle of neutrino mass physics. 
E. Grohs, G. M. Fuller, C. T. Kishimoto, and M. W. Paris
astro-ph.CO