Left: overall uncertainty (statistical and systematic) that is due to the considered variations in ΔY/ ΔZ, in dependence on the mass of the primary star. The lighter region shows the uncertainty on the median of the estimated β caused by ignoring the correct initial helium value. The intermediate colour regions show the overall error up to 1σ that is due to the cumulated contribution of observational errors and systematic bias as a result of the unknown initial helium value. The darker regions correspond to cumulated errors up to 2σ. Right: same as in the left panel, but in dependence on the relative age of the primary star.