Difference between the predicted (model) sound-speed profile and the profile derived from the inversion of helioseismic data (Sun), in the sense (Sun) – (model). The brown vertical bars show the errors on the inverted helioseismic data points, according to the results of Rhodes (1997). Errors have been multiplied by a factor of 10. The shaded band corresponds to the maximum uncertainty from our modelling of opacity and diffusion, while the continuous and dashed curves to a modified model computed with a fifth-force coupling of 𝒴 = 10⁻² and 10⁻⁴ respectively. Fifth-force models are computed on top of our reference GS98 model at standard gravity (see Table 2). The fifth-force effect in the radiative zone is clearly seen for 𝒴 = 10⁻², as expected from the model differences of Figs. 5 and 6.