  We present high-quality ground-based spectroscopic observations of 54
supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen
abundances 12 + log O/H between 7.1 and 8.3. We use the data to determine
abundances for the elements N, O, Ne, S, Ar and Fe. We also analyze Hubble
Space Telescope (HST) Faint Object Spectrograph archival spectra of 10
supergiant H II regions to derive C and Si abundances in a subsample of 7 BCGs.
The main result of the present study is that none of the heavy
element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O,
Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12 + log O/H < 7.6 (Z <
Zsun/20). This constancy implies that all these heavy elements have a primary
origin and are produced by the same massive (M > 10Msun) stars responsible for
O production. The dispersion of the C/O and N/O ratios in these galaxies is
found to be remarkably small, being only +/-0.03 dex and +/-0.02 dex
respectively. This very small dispersion is strong evidence against any
time-delayed production of C and primary N in the lowest-metallicity BCGs
(secondary N production is negligible at these low metallicities). The absence
of a time-delayed production of C and N is consistent with the scenario that
galaxies with 12 + log O/H < 7.6 are undergoing now their first burst of star
formation, and that they are therefore young, with ages not exceeding 40 Myr.
If very low metallicities BCGs are indeed young, this would argue against the
commonly held belief that C and N are produced by intermediate-mass (3Msun < M
< 9Msun) stars at very low metallicities, as these stars would not have yet
completed their evolution in these lowest metallicity galaxies.
