  We calculate and display the primordial light-element abundances as a
function of a neutrino degeneracy parameter \xi common to all flavors. It is
the only unknown parameter characterizing the thermal medium at the primordial
nucleosynthesis epoch. The observed primordial helium abundance Y_p is the most
sensitive cosmic ``leptometer.'' Adopting the conservative Y_p error analysis
of Olive and Skillman implies -0.04 \alt \xi \alt 0.07 whereas the errors
stated by Izotov and Thuan imply \xi=0.0245+-0.0092 (1 sigma). Improved
determinations of the baryon abundance have no significant impact on this
situation. A determination of Y_p that reliably distinguishes between a
vanishing or nonvanishing \xi is a crucial test of the cosmological standard
assumption that sphaleron effects equilibrate the cosmic lepton and baryon
asymmetries.
