We evaluate the effects that variations in He content have on bolometric corrections and T _ { eff } -colour relations .
To this aim , we compute ATLAS9 model atmospheres and spectral energy distributions for effective temperatures ranging from 3500 K to 40000 K for dwarfs and from 3500 K to 8000 K for giants , considering both “ He-non enhanced ” and “ He-enhanced ” compositions .
The considered variations in He content are of \Delta Y = +0.1 and +0.2 for the metallicity \mbox { [ { M } / { H } ] } = +0.5 and \Delta Y = +0.1 for \mbox { [ { M } / { H } ] } = -0.5 and -1.5 .
Then , synthetic photometry is performed in the UBVRIJHK system .
We conclude that the changes in bolometric corrections , caused by the adopted He-enhancements are in general too small ( less than 0.01 mag ) , for both dwarfs and giants , to be affecting present-day tables of bolometric corrections at a significant level .
The only possible exceptions are found for the U -band at T _ { eff } between 4000 K and 8000 K , where | \Delta BC _ { U } | amounts to \sim 0.02 mag , and for T _ { eff } equal to 3500 K , where | \Delta BC _ { S _ { \lambda } } | values become clearly much higher ( up to 0.06 mag for passbands from U to V ) .
However , even in the latter case the overall uncertainty caused by variations in the He content may be not so significant , because the ATLAS9 results are still approximative at their lowest temperature limit .