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Spectroscopic observations obtained with the VLT of one planetary nebula (PN)in Sextans A and of five PNe in Sextans B and of several HII regions (HII) inthese two dwarf irregular galaxies are presented. The extended spectralcoverage, from 320.0 to 1000.0nm, and the large telescope aperture allowed usto detect a number of emission lines, covering more than one ionization stagefor several elements (He, O, S, Ar). The electron temperature (Te) diagnostic[OIII] line at 436.3 nm was measured in all six PNe and in several HII allowingfor an accurate determination of the ionic and total chemical abundances bymeans of the Ionization Correction Factors method. For the time being, thesePNe are the farthest ones where such a direct measurement of the Te isobtained. In addition, all PNe and HII were also modelled using thephotoionization code CLOUDY. The physico-chemical properties of PNe and HII arepresented and discussed. A small dispersion in the oxygen abundance of HII wasfound in both galaxies: 12 + $\log$(O/H)=7.6$\pm$0.2 in SextansA, and7.8$\pm$0.2 in SextansB. For the five PNe of SextansA, we find that 12 +$\log$(O/H)=8.0$\pm$0.3, with a mean abundance consistent with that of HII. Theonly PN known in SextansA appears to have been produced by a quite massiveprogenitor, and has a significant nitrogen overabundance. In addition, itsoxygen abundance is 0.4 dex larger than the mean abundance of HII, possiblyindicating an efficient third dredge-up for massive, low-metallicity PNprogenitors. The metal enrichment of both galaxies is analyzed using these newdata.Comment: 19 pages, 11 figures, Accepted by A&

The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra