SIMBAD references

We use a sample of 536 H II regions located in nearby spirals, with a homogeneous determination of their T_e-based abundances, to obtain new empirical calibrations of the N2O2, N2S2, O3N2, and N2 strong-line indices to estimate the nitrogen-to-oxygen abundance ratio when auroral lines are not detected. All indices are strongly correlated with the T_e-based log (N/O) for our H II region sample, even more strongly than with 12 + log (O/H). N2O2 is the most strongly correlated index, and the best fit to the log (N/O)-N2O2 relation is obtained with a second-order polynomial. The derived relation has a low dispersion (rms < 0.09 dex), being valid in the range -1.74 < N2O2 < 0.62 (or -1.81 < log (N/O) < -0.13). We have compared our calibration with previous ones and have discussed the differences between them in terms of the nature of the objects used as calibrators.