  Considering the gauge model SU(2)_L x U(1) with the extension of the lepton
sector by right-handed sterile Majorana neutrinos, we distinguish a "minimal
parametric mixing scenario" for a complete matrix of 6 x 6 light (active) and
sterile neutrinos, which depends, in addition to experimentally measured
physical parameters, only on the masses of sterile neutrinos. For such a
scenario, improved cosmological bounds are provided, resulting from the
lifetime of sterile neutrinos and the fraction of energy carried by sterile
neutrino dark matter.
