  New results, namely the independent determination of the deuterium abundance
in several quasar absorption systems, and the complementary determination of
the cosmological baryon density by observations of anisotropies in the cosmic
microwave background (CMB), allow for a reevaluation of the constraints on the
relativistic particle content of the universe at primordial nucleosynthesis.
Expressed in terms of the neutrino energy density, we find 1.7 < N_\nu < 3.5
(95% CL). In particular, we show that phenomenological four neutrino models
including a sterile state (not participating in SU(2)_L x U(1)_Y interactions)
unavoidably thermalize a fourth neutrino, and are highly disfavored in the
standard minimal model of primordial nucleosynthesis, if the systematic
uncertainty in the primordial helium abundance is small. We describe plausible
extensions of the minimal model which evade this constraint.
