Chemical abundances in 7 metal-poor HII regions and a determination of
  the primordial helium abundance
  We conducted a long-slit spectrophotometry analysis to obtain the chemical
abundances of seven metal-poor HII regions in three galaxies: UM 160, UM 420,
and TOL 0513-393. The data have been taken with the Focal Reducer Low
Dispersion Spectrograph 1 (FORS1) at the 8.2-m Very Large Telescope. We derived
the physical conditions and the chemical abundances of N, O, Ne, S, Ar, and Cl.
We also performed a detailed analysis that involves abundance determinations
using the $t^2$ formalism. Based on HeI recombination line intensity ratios,
together with the Helio14 code, we derived the abundance of He. In addition,
for a value $\Delta Y/\Delta Z_O =3.3\pm 0.7$, we have estimated that the
primordial helium abundance by mass is $Y_{\rm P}=0.2448\pm0.0033$. This value
agrees with values derived from Standard Big Bang Nucleosynthesis and with
other recent determinations of $Y_{\rm P}$.
