Primordial helium fraction Y_He as a function of λ_G². For each curve, the baryon physical density, ρ_b, which is proportional to ω_b/λ_G², has been kept constant while λ_G varies. The grey band indicates the observational constraints of Olive (2021). The black star is the best-fit standard ΛCDM model, while the red ellipse corresponds to the contour for the λ_GΛCDM model fitted with CMB + BAO from DESI + BBN. We observe that this contour is highly degenerate in the direction of constant physical density. The blue curve corresponds to the linear sensitivity model of Dent et al. (2007).