Left: evolutionary tracks for models of different masses (labelled by their solar masses) for Y_i = 0.245 and Z_i/X_i = 0.0005 plotted as a function of age. The dashed lines illustrate changing the M = 0.80M_⊙ model in Y_i (ΔY) and Z_i/X_i (ΔZ). The unreddened and reddened luminosity constraints are indicated by the error bars. Right: approximate correlations between M and the initial helium abundance Y_i assuming an age >12 Gyr (continuous) and then extending it to >10 Gyr (dashed), given the luminosity constraint. See Sect. 7.2 for details.