  Local star-forming galaxies show properties that are thought to differ from
galaxies in the early Universe. Among them, the ionizing stellar populations
and the gas geometry make the recipes designed to derive chemical abundances
from nebular emission lines to differ from those calibrated in the Local
Universe. A sample of 1969 Extreme Emission Line Galaxies (EELGs) at a redshift
0 < z < 0.49, selected from the Sloan Digital Sky Survey (SDSS) to be local
analogues of high-redshift galaxies, was used to analyze their most prominent
emission lines and to derive total oxygen abundances and nitrogen-to-oxygen
ratios following the direct method in the ranges 7.7 < 12+log(O/H) < 8.6 and
-1.8 < log(N/O) < -0.8. They allow us to obtain new empirically calibrated
strong-line methods and to evaluate other recipes based on photoionization
models that can be later used for a chemical analysis of actively star-forming
galaxies in very early stages of galaxy evolution. Our new relations are in
agreement with others found for smaller samples of objects at higher redshifts.
When compared with other relations calibrated in the local Universe, they
differ when the employed strong-line ratio depends on the hardness of the
ionizing radiation, such as O32 or Ne3O2, but they do not when the main
dependence is on the ionization parameter, such as S23. In the case of
strong-line ratios depending on [NII] lines, the derivation of O/H becomes very
uncertain due to the very high N/O values derived in this sample, above all in
the low-metallicity regime. Finally, we adapt the bayesian-like code
HII-Chi-mistry for the conditions found in this kind of galaxies and we prove
that it can be used to derive within errors both O/H and N/O, in consistency
with the direct method .
