ABSTRACT We conducted a long-slit spectrophotometry analysis to obtain the chemical abundances of seven metal-poor H ii 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 t2 formalism. Based on He i recombination line intensity ratios, together with the {\sc HELIO 14} code, we derived the abundance of He. In addition, for a value $\Delta Y/\Delta Z_O \rm =3.3\pm 0.7$, we have estimated that the primordial helium abundance by mass is YP = 0.2448 ± 0.0033. This value agrees with values derived from Standard Big Bang Nucleosynthesis and with other recent determinations of YP.

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