tokens: atmospheres with the classical 1D approach we can calculate abundance corrections and abundance errors. Given a measured LTE abundance, we define the abundance correction as the abundance difference compared to LTE, such that the non-LTE equivalent width matches the LTE equivalent width. 3 Results 3.1 Departure coefficients In order to understand the behaviour of the abundance corrections and , we can look at the variation of the departure coefficients, b ( τ R ) = n NLTE ∕ n LTE , for the 1D hydrostatic models and 2D models. For the first, b 1D values were calculated for the lower 3s 5 S and upper 3p 5 P levels of the O I 777 nm triplet in the 4D parameter space τ R , T eff , log g eff , ξ t . For each combination a cubic interpolation that splits the multi-dimensional case into a sequence of 1D interpolations was performed on the grid of 1D models to obtain departures from LTE as functions of the Rosseland optical depth. In the 2D models – due to the horizontal inhomogeneities produced by convection – departures from LTE b ( τ R ) are different for in- and outflows (e.g. Amarsi et al. 2016 ). In order to compare departures from LTE for 2D models with the 1D case, b 2D values for each individual 2D snapshot were horizontally averaged over surfaces of constant optical depth. Both 2D models predict a similar dependence of the abundance corrections on effective temperature. Since they are similar we focused on the nine-day model, and calculated median 2D departures from LTE. We plot the departure coefficients in Fig. 3 . Namely, the median 2D departures from LTE as a function of τ R and photometricphase for the lower and upper levels are shown by solid blue and red lines, respectively. One-dimensional interpolated b 1D departures from LTE are shown by dashed lines using the same colours for the levels. Qualitatively, the departures from LTE of 1D and 2D models show a similar dependence on the photometric phase. The lower and upper levels of the O I 777 nm triplet, namely 3s 5 S and 3p 5 P, are almost always overpopulated during the pulsating cycle. At photometric phases φ ph ≈ 0.3...0.8 the lower and upper levels are less underpopulated compared to the rest of the phases, due to the minimum on the light curve and effective temperature. Quantitatively, for this range of phases 1D and 2D models predict very similar optical depth dependences of the departures from LTE. As a result, the abundance correction is small. Moreover, the 2D non-LTE versus 1D LTE abundance correction turns out to be smallest, as shown in Sect. 3.5 . 3.2 O I 777 nm triplet profiles We illustrate the temporal evolution (over a pulsation cycle) of the normalised emergent LTE and non-LTE O I 777 nm triplet flux profiles in Fig. 4 for M3 and M9. The line profiles are shown without adding instrumental broadening or noise. This allows us to identify the occasional multi-component structure of the line profiles, for example at photometric phases φ ph = 0.299 and 0.318 for M3 and
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