tokens: metallicity than QGs-A. The median stacked spectra, corrected for dust extinction, of QGs-A and QGs-B have a blue stellar continuum, suggesting a young mean stellar population. The analysis confirms a weak [] emission and that also other high ionization emission lines (such as []) are weak in both QGs stacked spectra, and the [] /H α ratios are consistent with a low ionization level. On the contrary, [], H α and H β (i.e. low ionization lines) are still strong and with an Hα flux slightly weaker () than the one of SF galaxies. We find that QGs have D n 4000 and EW(H α) values intermediate between SF and already quenched galaxies, confirming that they have just stopped their star formation and have a young/intermediate mean stellar population. In the colour-mass diagram, the bulk of the QGs-A and QGs-B resides in the blue cloud region, and only few of them (∼3 per cent) lie in the green valley region. The QGs have masses log( M /M ⊙ ) > 9.5, comparable with those of the SF population, being the QGs-B, on average, more massive. This suggests that, as expected in a downsizing scenario, star formation quenching has not started yet for low-mass galaxies, consistently with a lack of low-mass red galaxies. Their H α emission is compatible with a just quenched SFR of the order of 0.6–10 M ⊙ yr −1 , similar to that of the SF main sequence population, suggesting the presence of residual ionized gas in our QGs. The emission from the median stacked spectra is, however, weaker than that in the SF population, suggesting that the depletion of the gas has started. The morphologyand concentration index (C=R90/R50) of QGs are similar to those of the SF population, suggesting that no morphological transformation has occurred yet, in the early phase after the quenching of the star formation. However, some of them have a concentration index higher than the threshold that divides the early-type from the other galaxy types (i.e. C > 2.6) and they could have experienced morphological transformation during the interruption of the star formation. Compared to the parent SF population, we find an excess of QGs in high-density environments (∼42 per cent), in particular for QGs-B. QGs in high-density environments are preferentially satellites (from ∼60 to 80 per cent). Approximatively 5 per cent of QGs are in groups/clusters which have more than 30 members, but no one of them is the brightest galaxy in its environments. From the fraction of QG candidates ( of the SF population) we estimate the quenching time-scales for these populations to be between 10 and 18 Myr. These values are compatible with the sharp quenching models by C17 . If we assume that at most 50 per cent the entire SF-[]undet population (∼7.5 per cent) is in a low ionization state, as witnessed by the low but not extreme [] /H α ratio in their stacked spectrum, we obtain an upper limit to the quenching time-scale of ≃ 0.76 Gyr. In this case, the quenching time-scale is compatible with galaxies which are
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