  We present a new determination of the primordial helium mass fraction Yp,
based on 93 spectra of 86 low-metallicity extragalactic HII regions, and taking
into account the latest developments concerning systematic effects. These
include collisional and fluorescent enhancements of HeI recombination lines,
underlying HeI stellar absorption lines, collisional and fluorescent excitation
of hydrogen lines and temperature and ionization structure of the HII region.
Using Monte Carlo methods to solve simultaneously for the above systematic
effects, we find the best value to be
Yp=0.2565+/-0.0010(stat.)+/-0.0050(syst.). This value is higher at the 2sigma
level than the value given by Standard Big Bang Nucleosynthesis (SBBN),
implying deviations from it. The effective number of light neutrino species Nnu
is equal to 3.68^+0.80_-0.70 (2sigma) and 3.80^+0.80_-0.70 (2sigma) for a
neutron lifetime tau(n) equal to 885.4+/-0.9 s and 878.5+/-0.8 s, respectively,
i.e. it is larger than the experimental value of 2.993+/-0.011.
