Comparison between the effect of a modification of the EoS at standard gravity, and that of the fifth force at fixed EoS on the sound-speed profile. The variation δc_s is in the sense (reference model) – (modified model), and the vertical axis is on natural logarithmic scale. Our reference model uses the mixed OPAL/Free EoS, whereas the alternative EoS’s at standard gravity are based on pure Free EoS (‘modified EoS 1’), and pure OPAL (‘modified EoS 2’) respectively. For more details we refer to Sect. 4. The models with fifth force in the figure are computed on top of our reference EoS. We remind that the fifth-force effect on the sound-speed profile within the radiative zone peaks around r ≃ 0.25 R_⊙ (see also Figs. 5 and 6).