We study the effect that the temperature structure has on the determination of the primordial helium abundance , Y _ { p } .
We provide an equation linking T ( O iii ) , the temperature derived from the [ O iii ] lines , and T ( He ii ) , the temperature of the He I lines , both for H ii regions with O ^ { + + } only and for H ii regions where a fraction of O ^ { + } is present .
By means of T ( He ii ) , which is always smaller than T ( O iii ) , we derive the helium abundances of 5 objects with low and very low metallicity ( NGC 346 , NGC 2363 , Haro 29 , SBS 0335–052 , and I Zw 18 ) ; these objects were selected from the literature because they include the 3 low metallicity objects with the best line determinations and the 2 objects with the lowest metallicity .
From these abundances we obtain that Y _ { p } ( nHc ) = 0.2356 \pm 0.0020 , a value 0.0088 lower than that derived by using T ( O iii ) .
We call this determination Y _ { p } ( nHc ) because the collisional contribution to the Balmer line intensities has not been taken into account .
All the recent Y _ { p } determinations in the literature have not taken into account the collisional contribution to the Balmer line intensities .
By considering the collisional contribution to the Balmer line intensities of these five objects we derive that Y _ { p } ( +Hc ) = 0.2384 \pm 0.0025 .