Left: mean cluster [Na/Fe] ratio, computed using only the member stars, as a function of stellar mass. The new observations are shown as red diamond symbols. The typical standard deviation of the mean [Na/Fe], ±0.05 dex, is shown as an error bar. The data of the Gaia-ESO clusters are from Smiljanic et al. (2016), Overbeek et al. (2017), and Tang et al. (2017). The data of the Hyades are from Smiljanic (2012). We estimate the uncertainty in the turn-off masses to be less than ±0.2 M⊙. Right: [Na/Fe] as a function of mass for the sample of selected Cepheids. Two Cepheids suspected to be at the first crossing are shown as red stars; the other Cepheids are indicated as solid circles. The abundances are in LTE. References are given in the text. The models extend only up to 6 M⊙.
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