Same observations as in Fig. 1. Top: raw autocorrelator counts of the two phases re-aligned on top of each other and averaged together. A polynomial of 25th order is fitted (in red). Red vertical lines delimit the weak telluric and strong astrophysics ¹²CO lines. Middle: baseline-subtracted raw average. Bottom: comparison between the standard data reduction of the observations (black) and TP mode reduction of the data (red).