Constraints in the ω_b–N_eff plane from Planck TT,TE,EE+lowE and Planck TT,TE,EE+lowE+BAO+lensing data (68% and 95% contours) compared to the predictions of BBN combined with primordial abundance measurements of helium (Aver et al. 2015, in grey) and deuterium (Cooke et al. 2018, in green and blue, depending on which reaction rates are assumed). In the CMB analysis, N_eff is allowed to vary as an additional parameter to the base-ΛCDM model, while Y_P is inferred from ω_b and N_eff according to BBN predictions. For clarity we only show the deuterium predictions based on the PArthENoPEcode with two assumptions on the nuclear rate d(p, γ)³He (case (a) in blue, case (b) in green). These constraints assume no significant lepton asymmetry.