GS133 spectrum showing tentative Balmer absorption detection in the vicinity of Hα and [N II] emission lines. The black curve shows the integrated spectrum in velocity space, with v = 0 km s⁻¹ at the position of the Hα line. The red curve (with 1σ uncertainties in light-red) displays the residuals obtained by subtracting the best-fit Gaussian emission lines presented in Fig. 2. Two low S/N absorption features are detected close to the orange vertical lines, which mark the velocities of the mini-BAL features observed in the UV (see Table 2).