Evolution of Γ₁ as a function of the reduced radius in the superficial layers of stars of fixed large separation. Every star has a mass of 1.052 M_⊙. Two families of curves are displayed. The black ones have a fixed value of the ratio between the initial hydrogen and heavy elements abundances of (Z/X)₀ = 0.022 and variable initial helium abundance. The red ones have a fixed initial helium abundance of Y₀ = 0.24 and a variable initial heavy elements abundance. The different values are displayed on the figure.