Constraints in the ω_b–N_eff plane from Planck and Planck+BAO data (68% and 95% contours) compared to the predictions of BBN, given primordial element abundance measurements. We show the 68% and 95% confidence regions derived from ⁴He bounds compiled by Aver et al. (2013) and from deuterium bounds compiled by Cooke et al. (2014). In the CMB analysis, N_eff is allowed to vary as an additional parameter to base ΛCDM, with Y_P fixed as a function of ω_b and N_eff, according to BBN predictions. These constraints assume no significant lepton asymmetry.