Four-Neutrino Mixing, Oscillations and Big-Bang Nucleosynthesis
  We investigate the implications of the standard Big-Bang Nucleosynthesis constraint on the number of light neutrinos in the framework of the two four-neutrino schemes that are favored by the results of neutrino oscillation experiments. 
S.M. Bilenky, C. Giunti, W. Grimus, T. Schwetz