Expected anti-correlation between helium (in mass fraction) and [O/Na] for second generation stars (at birth) born out of material ejected through slow winds of massive stars and diluted with gas of pristine composition. The tracks correspond to the cases of 20 and 120 M_⊙ polluter stars (dashed and full lines, respectively) assuming different values (0.2 and 0.8) for the minimum dilution coefficient. See text for more details. The shaded regions indicate the star-to-star abundance differences derived in Sect. 3.5 for #5644 and #14 565 and related uncertainties (Δ [O/Na]  = 0.65 and ΔY < 0.07).