Primordial helium abundance determination using sulphur as metallicity
  tracer
  The primordial helium abundance $Y_P$ is calculated using sulphur as
metallicity tracer in the classical methodology (with $Y_P$ as an extrapolation
of $Y$ to zero metals). The calculated value, $Y_{P,\,S}=0.244\pm0.006$, is in
good agreement with the estimate from the Planck experiment, as well as,
determinations in the literature using oxygen as the metallicity tracer. The
chemical analysis includes the sustraction of the nebular continuum and of the
stellar continuum computed from simple stellar population synthesis grids. The
$S^{+2}$ content is measured from the near infrared
$\left[SIII\right]\lambda\lambda9069\AA,9532\AA$ lines, while an
$ICF\left(S^{3+}\right)$ is proposed based on the $Ar^{3+}/Ar^{2+}$ fraction.
Finally, we apply a multivariable linear regression using simultaneously
oxygen, nitrogen and sulphur abundances for the same sample to determine the
primordial helium abundance resulting in $Y_{P-O,\,N,\,S}=0.245\pm0.007$
