We use spectroscopic observations of a sample of 82 H ii regions in 76 blue compact galaxies to determine the primordial helium abundance Y _ { p } and the slope dY / dZ from the Y – O/H linear regression .
To improve the accuracy of the dY / dZ measurement , we have included new spectrophotometric observations of 33 H ii regions which span a large metallicity range , with oxygen abundance 12 + log ( O/H ) varying between 7.43 and 8.30 ( Z _ { \odot } /30 \leq Z \leq Z _ { \odot } /4 ) .
Most of the new galaxies were selected from the First Byurakan , the Hamburg/SAO and the University of Michigan objective prism surveys .
For a subsample of 7 H ii regions , we derive the He mass fraction taking into account known systematic effects , including collisional and fluorescent enhancements of He i emission lines , collisional excitation of hydrogen emission , underlying stellar He i 
absorption and the difference between the temperatures T _ { e } ( He ii ) in the He ^ { + } zone and T _ { e } ( O iii ) derived from the collisionally excited [ O iii ] lines .
We find that the net result of all the systematic effects combined is small , changing the He mass fraction by less than 0.6 % .
By extrapolating the Y vs. O/H linear regression to O/H = 0 for 7 H ii regions of this subsample , we obtain Y _ { p } = 0.2421 \pm 0.0021 and dY / d O = 5.7 \pm 1.8 , which corresponds to dY / dZ = 3.7 \pm 1.2 , assuming the oxygen mass fraction to be O=0.66 Z .
In the framework of the standard Big Bang nucleosynthesis theory , this Y _ { p } 
corresponds to \Omega _ { b } h ^ { 2 } = 0.012 ^ { +0.003 } _ { -0.002 } , where h is the Hubble constant in units of 100 km s ^ { -1 } Mpc ^ { -1 } .
This is smaller at the 2 \sigma level than the value obtained from recent deuterium abundance and microwave background radiation measurements .
The linear regression slope dY / d O = 4.3 \pm 0.7 ( corresponding to dY / dZ = 2.8 \pm 0.5 ) for the whole sample of 82 H ii regions is similar to that derived for the subsample of 7 H ii regions , although it has a considerably smaller uncertainty .