custom-made flat-field is required.[^1] For each image, we calculate the reference position of the star using an image taken without the dispersive element. This reference position and the beam angle are used to calculate the wavelength at each pixel, using the grism’s nominal dispersion relation [@Pirzkal2009a]. Then, the response of each pixel as a function of wavelength is determined by evaluating a 4[$^{\textrm{\scriptsize th}}$ ]{}-order polynomial to flat-fields taken at 10 different wavelengths. A flat-field is made for each image, and an average flat-field is built for each visit. The latter one is used for the calibration. Images of a given visit are thus calibrated using the same flat-field.

White-light photometry {#sec:White-light photometry}
======================

Lightcurve extraction {#sec:Lightcurve extraction}
---------------------

In order to derive the star and planet parameters, we perform white-light photometry by summing the flux over the spectral trace. We optimize the photometric apertures for the star and sky background. We extract and compare lightcurves for apertures of different sizes, varying between 6 and 50 pixels in $y$. The sky background is computed from the same quadrant as the spectral trace, either above or below depending on the position of the star; the width of the sky region varies 