Surface abundance of [C/N] as a function of stellar ages (left panels) and stellar masses (right panels), colour-coded by metallicity, for a synthetic thin disc computed with the BGM. Stars have been selected to be in the clump according to their ΔΠ_{ℓ = 1}, as well as before and after the RGB-bump according to their log(g) values. Simulations including or not the effect of thermohaline instability are shown as colour-dots and grey crosses respectively.