Mass-metallicity relation (MZR) for our sample. The metallicities are derived exploiting the Curti et al. (2017, C17) calibration of the N2 indicator (for J1038, J1958 and J0143 we reported in the N2+R3 values, affected by smaller uncertainties; see Table 4). Previous N2-based metallicities from the literature are plotted with different colours/symbols as reported in the legend. The grey shaded area encompass the 1σ dispersion of the local median MZR in bins of stellar mass, as derived from the C17 calibrations for SDSS galaxies (Curti et al., in prep.). The T_e-based local MZR derived from staked spectra in bins of stellar mass by Andrews & Martini (2013) is also shown as the dashed green line. We applied corrections to the stellar mass scale to account for the different IMFs adopted by previous works. For J1038 and J1958, integrated spectra over the knots associated with an individual emitting source have been used to maximise the S/N in the faint [N II] emission lines (see Sects. 4.1 and 4.2 for details). Following Jones et al. (2013), for the J1038 system we distinguished between the A1 image and the other knots, possibly associated with two interacting galaxies of 10^9.9 M_⊙ and ∼ 10^9.1 M_⊙, respectively.