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We present deep spectrophotometry of several Hii regions in the nearbylow-mass spiral galaxies NGC 300 and M33. The data have been taken with UVESand OSIRIS spectrographs attached to the 8m VLT and 10.4m GTC telescopes,respectively. We have derived precise values of the physical conditions foreach object making use of several emission line-intensity ratios. Inparticular, we have obtained direct determinations of the electron temperaturein all the observed objects. We detect pure recombination lines (RLs) of Ciiand Oii in several of the Hii regions, permitting to derive their C/H and C/Oratios. We have derived the radial abundance gradient of O for each galaxymaking use of collisionally excited lines (CELs) and RLs, as well as the C andN gradients using RLs and CELs, respectively. We obtain the first determinationof the C/H gradient of NGC 300 and improve its determination in the case ofM33. In both galaxies, the C/H gradients are steeper that those of O/H, leadingto negative C/O gradients. Comparing with similar results for other spiralgalaxies, we find a strong correlation between the slope of the C/H gradientand M_V. We find that some Hii regions located close to the isophotal radius(R_25) of NGC 300 and M33 show C/O ratios more similar to those typical ofdwarf galaxies than those of Hii regions in the discs of more massive spirals.This may be related to the absence of flattening of the gradients in theexternal parts of NGC 300 and M33. Finally, we find very similar N/H gradientsin both galaxies and a fair correlation between the slope of the N/H gradientand M_V comparing with similar data for a sample of spiral galaxies.

Carbon and oxygen abundance gradients in NGC300 and M33 from optical recombination lines