Comparison between synthetic and observed hydrogen Balmer line profiles. Top: NLTE (black) and LTE (red) theoretical transmission spectra compared to the observed Hα line profile (blue dots) presented by Damasceno et al. (2024). The green line is a Gaussian fit to the observed line profiles. The black and red dashed lines correspond, respectively, to the NLTE and LTE line profiles convolved with a Gaussian profile with a width such that the height of the lines equals the height of the Gaussian fit to the observations. Middle: same as top panel but for the Hβ line. Bottom: comparison of the observed Hα line (blue dots) with NLTE synthetic spectra computed with the system parameters listed in Table 2 (black; same as in the top panel), with M_p = 2.0 M_J (red) and 2.5 M_J (magenta), and with T_eff = 8600 K (orange).