2D joint posterior distribution for N_eff and Y_P with both parameters varying freely, determined from Planck+WP+highL data. Samples are colour-coded by the value of the angular ratio θ_D/θ_∗, which is constant along the degeneracy direction. The N_eff-Y_P relation from BBN theory is shown by the dashed curve. The vertical line shows the standard value N_eff = 3.046. The region with Y_P> 0.294 is highlighted in grey, delineating the region that exceeds the 2σ upper limit of the recent measurement of initial Solar helium abundance (Serenelli & Basu 2010), and the blue horizontal region is the 68% confidence region from the Aver et al. (2012) compilation of ⁴He measurements.