Chemistry in the dIrr galaxy Leo A
  We present chemical abundance determinations of two H II regions in the dIrr
galaxy Leo A, from GTC OSIRIS long-slit spectra. Both H II regions are of low
excitation and seem to be ionised by stars later than O8V spectral type. In one
of the H II regions we used the direct method: O$^{+2}$ ionic abundance was
calculated using an electronic temperature determined from the [O III]
$\lambda\lambda$4363/5007 line ratio; ionic abundances of O$^+$, N$^+$, and
S$^+$ were calculated using a temperature derived from a parameterised formula.
O, N and S total abundances were calculated using Ionisation Correction Factors
from the literature for each element. Chemical abundances using strong-line
methods were also determined, with similar results. For the second H II region,
no electron temperature was determined thus the direct method cannot be used.
We computed photoionisation structure models for both H II regions in order to
determine their chemical composition from the best-fitted models. It is
confirmed that Leo A in a very low metallicity galaxy, with
12+log(O/H)=7.4$\pm$0.2, log(N/O)=$-$1.6, and log(S/O)=$-$1.1. Emission lines
of the only PN detected in Leo A were reanalysed and a photoionisation model
was computed. This PN shows 12+log(O/H) very similar to the ones of the H II
regions and a low N abundance, although its log(N/O) ratio is much larger than
the values of the H II regions. Its central star seems to have had an initial
mass lower than 2 M$_\odot$.
