Top: Eddington factor at the ZAMS for non-rotating stars at a metallicity Z = 10^-3.5 ≃ Z_⊙/ 50. Bottom: Initial conditions for the occurrence of CHE in single stars at the same metallicity, in terms of mass at the ZAMS and initial ratio of angular frequency Ω to its critical value Ω_crit at the surface. The coloured region indicates stars for which the surface helium abundance at TAMS exceeds 0.8 and is a sharp transition. Solid lines correspond to a fixed initial rotational period, while dashed lines indicate the value of Ω/Ω_crit if the star is the primary of a binary system at a fixed mass ratio q which exactly fills its Roche lobe at the ZAMS (see Eq. (9)).