SIMBAD references

We studied the ionized gas spectra of star-forming regions in the Holmberg II galaxy using optical long-slit spectroscopic observations made with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS). We estimated the oxygen, nitrogen, sulphur, neon and argon abundances in individual H ii regions and found the average metallicity in the galaxy to be Z ≃ 0.1 or 0.3Z☉, depending on the estimation method employed. We used these observations combined with the results of our earlier studies of the irregular galaxy IC 10 and the blue compact dwarf galaxy VII Zw 403 to compare the currently most popular methods of gas metallicity estimation in order to select from among them the techniques that are most reliable for analysing irregular galaxies. To this end, we used the `direct' Te method and six empirical and theoretical methods. The results of our observations mostly confirmed the conclusions of López-Sánchez et al. based on the analysis of systematic deviations of metallicity estimates derived by applying different methods to `model' H ii regions.