spectral trace, after subtracting the sky background in that column. The bounds in the $y$ direction of the spectral trace and the sky region are the same as in the white-light analysis Section \[sec:Lightcurve extraction\]. These spectra are affected by strong variations due to the grism response in wavelength: for example, 15% between 1.54 and 1.58 $\mu$m (Figure \[fig:rawspec\]). We estimate the influence of the sky background. The median sky background is 27 [e$^{-}$]{}/px, which is $5.4\times10^{-4}$ lower than the median flux pixels inside the stellar region. A determination of sky background in the whole image rather than in each column leads to no significant changes.

A common method to extract the planetary spectrum is to divide an in-transit by an out-of-transit stellar spectrum. To empirically infer the uncertainties of this method for our data, we use all possible pairs of orbits for each star and each visit including pairs of out-of-transit orbits, but as before, discarding the first orbit of each visit. We do not use the ingress and egress. The resulting spectra have strong variations, comprised between 0.3 and 3% peak-to-peak (Figure \[fig:rawspec-div\]). For comparison, the precision of the Poisson noise limited spectrum would be 370 ppm. Evidently, 