The He-4 abundance in the metal-deficient blue compact dwarf galaxies Tol 1214-277 and Tol 651

We present high-quality Keck telescope spectroscopic observations of the tw o metal-deficient blue compact dwarf (BCD) galaxies Tol 1214-277 and Tol 65 . These data are used to derive the heavy-element and helium abundances. We find that the oxygen abundances in Tol 1214-277 and Tol 65 are the same, 1 2 + log (O/H) = 7.54 +/- 0.01, or Z./24, despite the different ionization c onditions in these gal axies. The nitrogen-to-oxygen abundance ratio in bot h galaxies is log (N/O) = -1.64 +/- 0.02 and lies in the narrow range found for the other most metal-deficient BCDs. We use the five strongest He I em ission lines lambda lambda 3889, 4471, 5876, 6678, and 7065 to self-consist ently correct their intensities for collisional and fluorescent enhancement mechanisms and to derive the He-4 abundance. Underlying stellar absorption is found to be important for the He I lambda 4471 emission line in both ga laxies, being larger in Tol 65. The weighted He-4 mass fractions in Tol 121 4-277 and Tol 65 are, respectively, Y = 0.2458 +/- 0.0039 and 0.2410 +/- 0. 0050 when the three He I emission lines lambda lambda 4471, 5876, and 6678 are used and are, respectively, Y = 0.2466 +/- 0.0043 and 0.2463 +/- 0.0057 when the He I lambda 4471 emission line is excluded. These values are in v ery good agreement with recent measurements of the 4He mass fraction in oth ers of the most metal-deficient BCDs by Izotov et al. We find that the comb ined effect of the systematic uncertainties due to the underlying He I stel lar absorption lines, ionization, and temperature structure of the H II reg ion and collisional excitation of the hydrogen emission lines is likely sma ll, not exceeding similar to2% (the error is 2 sigma). Our results support the validity of the standard big bang model of nucleosynthesis.