Gradient profile as a function of mass fraction for a one-solar-mass CHeB star at solar metallicity and Y_c = 0.90. The helium profile is shown in parallel (black). The convective zone is in yellow; the overshooting layer is in green; and the sharp variation left by the “He flash” is in grey with XCO = 5%. The radiative gradient ∇_rad is in red, and the adiabatic gradient ∇_ad in blue. The real gradient ∇ is defined as follows: ∇ = ∇_ad in the COS zone, and ∇ = ∇_rad in the radiative zone. We focus on the central part of the star for illustrative purposes.