New calibrations for estimating the N/O ratio in HII regions
  We use a sample of 536 \hii\ regions located in nearby spirals, with an
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 \hii\ 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 ({\em 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.
