Previous authors have used the present mass density of the universe to constrain the masses of stable, left-handed neutrinos. We generalize these arguments to constrain the masses of stable, right-handed neutrinos which interact with strength GGF. Light neutrinos (1 MeV) are constrained to be less massive than 100 eV-2 keV, depending upon G and whether they are of the Dirac or Majorana type. Heavy neutrinos must be more massive than (GFG) GeV, and less massive than about 10 TeV.
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