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We present deep spectrophotometry of 18 HII regions in the nearby massivespiral galaxies M101 and M31. We have obtained direct determinations ofelectron temperature in all the nebulae. We detect the CII 4267 line in severalHII regions, permitting to derive the radial gradient of C/H in both galaxies.We also determine the radial gradients of O/H, N/O, Ne/O, S/O, Cl/O and Ar/Oratios. As in other spiral galaxies, the C/H gradients are steeper than thoseof O/H producing negative slopes of the C/O gradient. The scatter of theabundances of O with respect to the gradient fittings do not support thepresence of significant chemical inhomogeneities across the discs of thegalaxies, especially in the case of M101. We find trends in the S/O, Cl/O andAr/O ratios as a function of O/H in M101 that can be reduced using Teindicators different from the standard ones for calculating some ionicabundances. The distribution of the N/O ratio with respect to O/H is ratherflat in M31, similarly to previous findings for the MilkyWay. Using the disceffective radius, Re, as a normalization parameter for comparing gradients, wefind that the latest estimates of Re for the Milky Way provide an excess ofmetallicity in apparent contradiction with the mass-metallicity relation; avalue about two times larger might solve the problem. Finally, using differentabundance ratios diagrams we find that the enrichment timescales of C and Nresult to be fairly similar despite their different nucleosynthetic origin.

Carbon, nitrogen and oxygen abundance gradients in M101 and M31