The O/H distribution in the low-mass galaxies NGC 2366 and NGC 4395

Results of a spectrophotometric survey in the magellanic barred galaxy NGC 2366 and in the small weakly barred spiral galaxy NGC 4395, employing imaging spectrophotometry with narrow-band interference filters in the lines of H alpha, H beta, [O III] lambda 5007 and [N II] lambda 6584, are presented. The use of [O III]/H beta as an abundance indicator is assessed; it is shown that for the probable range of the physical properties of the H II regions in NGC 2366 and NGC 4395, [O III]/H beta may not give reliable estimates of O/H abundances in the interstellar gas in these two low-mass galaxies. Instead we use [N II]/[O III], which is more dependable at relatively low abundances. The derived mean levels of O/H (+/-1 sigma dispersion) in NGC 2366 and NGC 4395 are 12 +/- log O/H = 8.19 +/- 0.14 and 8.33 +/- 0.25, respectively; the global gradients of both galaxies are flat. Mechanisms which could explain the absence of abundance gradients in low-mass and irregular galaxies are explored.