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tokens: of the A-AB distributions, a -value of for the A-B distributions, and a -value of for the AB-B distributions (from the AD-tests we obtain -values of , and for each bin, respectively). Finally, to analyse the distribution of slopes depending on the luminosity of the galaxies, we divided the sample again into three groups: luminous (L, mag), intermediate (I, mag), and faint (F, mag) galaxies. We obtained these results: The statistical tests yield a -value of % for the KS-test in the case with the largest differences, that is, when comparing luminous and intermediate galaxies. The remaining KS-tests return a -value of for the comparison between luminous and faint galaxies and a -value of when analysing the intermediate and faint distributions (a -value of , and respectively, for the AD-tests). Similar results for all these separations are found when using any scale-length normalisation for the radial distance instead of the disc effective radius, either the radius or the physical scale of the galaxy (i.e. the radial distance in kpc). 4.3. Common abundance gradient The fact that the distribution of the oxygen abundance gradients for all the galaxies in the sample is well fitted by a Gaussian function suggests the existence of a characteristic value for the slope, independent of other properties of the galaxies, as the morphological type or the luminosity. This is true when normalising the radial galactocentric distances to the disc effective radius and limiting the fitted interval between 0.5 and 2.0 . Below this range (), a nearly flat distribution or even a drop towards the centre is found for some galaxies of the sample. At larger galactocentric distances (), a flattening can be observed in the abundance gradient of most of the galaxies. It is easier to illustrate this result if we represent the radial distribution of the oxygen abundance for all the galaxies in the same figure. This is shown in the left panel of Fig. 8 . The black contours represent the density distribution of the star-forming spaxels. Although the radial gradient can be discerned through the contour plot, the wide range of abundances blurs the result. To clarify the origin of this widening, we colour-code each represented abundance according to the integrated stellar mass of the host galaxy in log scale. For clarity, only the oxygen abundance values with a contribution of at least 1% of the total number of spaxels are plotted. Adopting this scheme, it is evident that the abundances present a common radial gradient, but with an offset depending on the mass, as expected from the relation (and as discussed in Sect. 4.1 ). To remove this dependence on the mass from the map, we rescaled the oxygen abundances of each galaxy following the analytical form of the relation ( Tremonti et al. 2004 ) derived by Sánchez et al. (2013) , applying an offset according to the integrated stellar mass of the galaxy and normalising to the average value for the whole sample of 8.5. The