... spectra

Based on observations made with the Isaac Newton and William Herschel Telescopes.

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... abundance

The helium abundance is measured from models in which the fractional abundance by number $n_{\rm He}$ is specified. Several authors give the number ratio $y=\mbox{\,$n_{\rm He}$ }/\mbox{\,$n_{\rm H}$ }$instead. We give the latter in Tables 4 and 5 and elsewhere.

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... metallicity

Providing the stars are hydrogen-rich, the metallicity ( $\mbox{${\rm [Fe/H]}$ }$) represents the logarithm of the value relative to the solar metal abundance, assuming a solar distribution of elements heavier than helium.

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