Ground-based I_c -band differential photometry time series of a transit of TOI-1759 b obtained by the 0.4 m OAAlbanya observatory on May 20, 2020. The light blue points show relative fluxes of TOI-1759, and the dark blue points show weighted average bins with bin sizes of 0.01 days. The gray and green points show the relative fluxes for the comparison stars that we used in the differential photometry. The red line shows the best-fit transit model obtained from our analysis of the TESS data alone, as presented in Sect. 5.