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For nearby K dwarfs, the broadening of the observed main sequence (MS) at low metallicities is much narrower than expected from isochrones with the standard helium‐to‐metal enrichment ratio  Δ Y /Δ Z ∼ 2 . Though the latter value fits well the MS around solar metallicity, and agrees with independent measurements from H  ii  regions as well as with theoretical stellar yields and chemical evolution models, a much higher  Δ Y /Δ Z ∼ 10  is necessary to reproduce the broadening observed for nearby subdwarfs. This result resembles, on a milder scale, the very high  Δ Y /Δ Z   estimated from the multiple MSs in ω Cen and NGC 2808. Although not ‘inverted’ as in ω Cen, where the metal‐rich MS is bluer than the metal‐poor one, the broadening observed for nearby subdwarfs is much narrower than stellar models predict for a standard helium content. We use this empirical evidence to argue that a revision of lower MS stellar models, suggested from nearby stars, could significantly reduce the helium content inferred for the subpopulations of those globular clusters. A simple formula based on empirically calibrated homology relations is constructed for an alternative estimate of  Δ Y /Δ Z   in multiple MSs. We find that, under the most favourable assumptions, the estimated helium content for the enriched populations could decrease from   Y ≃ 0.4  to as low as   Y ≃ 0.3 .

Revisiting Δ Y /Δ Z from multiple main sequences in globular clusters: insight from nearby stars