We present the results of a joint investigation aimed at constraining the primordial He content ( Y _ { P } ) on the basis of both the Cosmic Microwave Background ( CMB ) anisotropy and two stellar observables , namely the tip of the Red Giant Branch ( TRGB ) and the luminosity of the Zero Age Horizontal Branch ( ZAHB ) .
Current baryon density estimates based on CMB measurements cover a wide range values 0.009 \la \Omega _ { b } h ^ { 2 } \la 0.045 , that according to Big Bang Nucleosynthesis ( BBN ) models would imply 0.24 \la Y _ { P } \la 0.26 .
We constructed several sets of evolutionary tracks and HB models by adopting Y _ { P } = 0.26 and several metal contents .
The comparison between theory and observations suggests that ZAHB magnitudes based on He-enhanced models are 1.5 \sigma brighter than the empirical ones .
The same outcome applies for the TRGB bolometric magnitudes .
This finding somewhat supports a Y _ { P } abundance close to the canonical 0.23-0.24 value .
More quantitative constraints on this parameter are hampered by the fact that the CMB pattern shows a sizable dependence on both Y _ { P } and the baryon density only at small angular scales , i.e .
at high l in the power spectrum ( l \ga 100 ) .
However , this region of the power spectrum could be still affected by deceptive systematic uncertainties .

Finally , we suggest to use the UV-upturn to estimate the He content on Gpc scales .
In fact , we find that a strong increase in Y _ { P } causes in metal-poor , hot HB structures a decrease in the UV emission .