Acoustic radius of the discontinuity related to the second helium ionization zone as a function of the global large separation. Clump stars are indicated by red diamonds and RGB stars by blue triangles. Error bars correspond to the 1σ uncertainties. The light green line indicates the theoretical acoustic radius of the second helium ionization zone for a 1 M_⊙ star during the RGB phase. The dark green line gives the same information for a 1.4 M_⊙ star.