Analysis steps shown for the Ca⁺ infrared triplet. a: combined transmission spectra of both time series in the rest frame of the star. The dark absorption feature is the planetary atmosphere. The bright emission-like feature is the residual originating from the spectra behind the planet during transit (Rossiter–McLaughlin effect). b: best fit for the stellar residual using the parameters in Table 3. c: combined transmission spectrum after dividing out the best fit for the stellar residual. d: panel c after correcting for systematic noise in the rest frame of the star (vertical de-trending). e: same as panel d, but in the rest frame of the planet. The vertical absorption features are the Ca⁺ lines of the planetary atmosphere.