Variation of the standard deviation (σ) of the residuals as a function of the number of principal components removed via PCA for the KELT-8 b (left panels) and the KELT-23 Ab (right panels) dataset. Top panels: relative variation ∆σ of the standard deviation of the residuals (expressed in percentage value and reported in logarithmic scale for better visualisation) in the different spectral orders (coloured solid and dashed lines). Bottom panels: relative variation of ∆σ (δ) in the different spectral orders (coloured solid and dashed lines). The solid black horizontal line represents the −0.1 threshold on δ used to identify the minimum number of principal components to select in order to remove the telluric and stellar contamination.