Relation between the X index for the strength of the He I line at 5876 Å (see Eq. (2) for a definition) and the He abundance by mass Y for BHB stars in NGC 2808 (Marino et al. 2014: open squares), NGC 6752 (Villanova et al. 2009: filled triangles), and M 4 (Villanova et al. 2012: filled squares). He abundances from these two last papers were corrected for departures from LTE and a zero point offset of 0.036 in Y. Overimposed is the best fit cubic through the origin (Eq. (3)).
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