Restrictions on neutrino oscillations from BBN. Non-resonant case
  New nucleosynthesis bounds on the oscillation parameters of active neutrinos mixed with a sterile one are derived for the non-resonant case. The controversy in the literature whether to use the annihilation rate or the total reaction rate for the estimates of sterile neutrino production is resolved in favor of the annihilation rate. In contrast to previous papers, the restrictions on oscillations of electronic neutrinos are weaker than those of muonic and tauonic ones. 
A.D. Dolgov