Two-dimensional representation of the MASCARA-2b spectra around Na I lines. The vertical axis of each panel represents the sequence number of the observed spectra (the time increases with the spectrum number); the red horizontal lines represent the limits between the in- and out-of-transit data. In the horizontal axis we present the wavelength in Å and the relative flux values are shown in the color bar. Panel a: original data after the normalization. Panel b: observed data after the telluric subtraction. Panels c, d, and e: residuals from the in-transit spectra and the master out spectrum ratio. The RM effect can be clearly observed in these panels. In panel d the data is binned by 20 pixels and in e we show the modeled RM residuals (without planetary absorption) obtained by applying to the modeled stellar spectra the same process as data from panels c and d. Panels f and g: residuals after correcting each single spectrum presented in c for the RM effect. We note in the process presented in Sect. 3 that the correction is applied to the final transmission spectrum after combining all In/M_out with the planet RV correction applied so as not to affect the planetary absorption. In panel g the data is binned by 20 pixels. The green vertical lines show the limits in wavelength of the RM effect (from −v sin i to +v sin i). Panel h: same as g decreasing the wavelength coverage and changing the contrast (see color-bar). Panels i: same as h after the planet RV correction. The blue vertical lines of panels h and i show the expected Na I position in wavelength due to the maximum and minimum RV of the planet during the transit (dashed line) and the mid-transit value, i.e., null RV (solid line).