Validity domain of the OPAL EOS (shaded area) in the plane (log    T, log    ρ); we superimposed the time evolution of the bottom of the atmosphere (τ_ph = 10) and the stellar centre for Z = 0.03 (thick solid line) and Z = 0.0002 (thick dashed line), and the whole structure at the beginning of the contraction along the Hayashi track (dotted line) and in the ZAMS (dot-dashed line) for Z = 0.0002.