Normalised flux of the master out in the wavelength range of the sodium doublet. The stellar sodium doublet of night one isshown in blue, of night two in orange. The inset shows a zoom into the line centre of the D2 line of the same data set. Night one shows the expected line shape of the stellar sodium doublet, however, night two shows a sodium emission at line centre (width approx. 6–8 bins), which has to be masked in each spectrum to avoid a false-positive detection of the stellar sodium feature as planetary sodium.