Scatter plot of the marginalised posterior distribution showing the oxygen mass as a function of the temperature for the last two observed spectra (the left-most trace corresponds to the very late time spectrum at +348 d). The colour denotes the LTE departure coefficient. The estimated value range for the LTE departure coefficient from the O I λ 7774 recombination line (Sect. C.1) is marked in the colour bar. One can see that the earlier spectrum provides a lower mass constraint, while the late one does not.