VLT spectroscopy of low-metallicity emission-line galaxies: abundance patterns and abundance discrepancies

International audienceContext. We present deep spectroscopy of a large sample of low-metallicity emission-line galaxies. Aims: The main goal of this study is to derive element abundances in these low-metallicity galaxies. Methods: We analyze 121 VLT spectra of H ii regions in 46 low-metallicity emission-line galaxies. Of these spectra 83 are archival VLT/FORS1+UVES spectra of H ii regions in 31 low-metallicity emission-line galaxies that are studied for the first time with standard direct methods to determine the electron temperatures, the electron number densities, and the chemical abundances. Results: The oxygen abundance of the sample lies in the range 12 + log O/H = 7.2-8.4. We confirm previous findings that Ne/O increases with increasing oxygen abundance, likely because of a higher depletion of oxygen in higher-metallicity galaxies. The Fe/O ratio decreases from roughly solar at the lowest metallicities to about one tenth of solar, indicating that the degree of depletion of iron into dust grains depends on metallicity. The N/O ratio in extremely low-metallicity galaxies with 12 + log O/H e derived from [O ii]lambda3727/(lambda7320 + lambda7330) or [S ii]lambda4068/(lambda6717 + lambda6730) and [O iii]lambda4363/lambda(4959+5007). The electron number densities Ne(Cl iii) and Ne(Ar iv) were derived in a number of objects and are found to be higher than Ne(O ii) and Ne(S ii). This has potential implications for the derivation of the pregalactic helium abundance. In a number of objects, the abundances of C++ and O++ could be derived from recombination lines. Our study confirms the discrepancy between abundances found from recombination lines (RLs) and collisionally excited lines (CELs) and that C/O increases with O/H