Heavy sterile neutrinos: Bounds from big-bang nucleosynthesis and SN   1987A
  Cosmological and astrophysical effects of heavy (10 - 200 MeV) sterile Dirac neutrinos, mixed with the active ones, are considered. The bounds on mass and mixing angle from both supernovae and big-bang nucleosynthesis are presented. 
A.D. Dolgov, S.H. Hansen, G. Raffelt, D.V. Semikoz