Top: Euler photometry (blue dots) taken with the Gunn r filter on the night of 2019 Feb 27. It is phase folded using GJ 436 b orbital period. The best fit model to the planetary transit and its 1σ uncertainty are depicted by the black line and the yellow area. Binned photometry (every 11 data points) is illustrated by the white dots. The starting time of the stellar flare seen in the ESPRESSO spectra is marked by the vertical orange line. Bottom: photometric residuals after removing the model from the observations.