Fig. 41.
Constraints on the helium abundance 
and number of effective neutrino species Neff from Planck TT,TE,EE+lowE and in combination with lensing and BAO. Results are consistent with the predictions of standard BBN (green line), and also the observed helium abundance (68% and 95% grey bands from Aver et al. 2015). The grey band at the top shows a conservative 95% upper bound inferred from the Solar helium abundance (Serenelli & Basu 2010). The black contours show the joint BBN-independent constraint from combining Planck TT,TE,EE+lowE+lensing+BAO and Aver et al. (2015).
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