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Fig. 9


a)Y – O/H for the sample of 1610 H ii regions. The five He i emission lines λ3889, λ4471, λ5876, λ6678, and λ7065 are used for the χ2 minimisation and the determination of Y. Large blue and green filled circles are for the HeBCD and VLT samples, small red filled circles are SDSS galaxies. We chose to let Te(He+) vary freely in the range 0.95−1.05 of the (He+) value. The continuous line represents the linear regression (whose equation is given at the bottom of the panel). b) Same as a), but only H ii regions with EW(Hβ) ≥ 100 Å are shown, with the excitation parameter x = O2+/O ≥ 0.7 and with 1σ error in Y ≤ 3%. The dotted line is the quadratic maximum-likelihood fit to the data. c) Distribution of the best derived values Te(He+)/Te(O iii) versus Te(O iii) for the case shown in b). d) Distribution of the best derived values Ne(He+) versus Ne(S ii) for the case shown in b).

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