J1727 spectroscopic database showing the BAs evolution during the outburst, zoomed into the Balmer $\mathrm{H}\delta$, $\mathrm{H}\gamma$, and $\mathrm{H}\beta$ transitions. The colour of the spectra corresponds to the X-ray spectral state, following the same convention as in Mata Sánchez et al. (2024): hard-intermediate state (HIMS, green), soft-intermediate state (SIMS, orange), and soft state (red). In addition, the low-luminosity transition between the soft and hard state is also marked for the new data (cyan). The best fit corresponding with the parameters shown in Fig. C.2 is shown as a solid blue line. The DIB next to $\mathrm{H}\beta$, as well as a nearby He I transition are shown in grey. The rest wavelength of the transition is marked with a black-dashed line, while blue-dotted lines mark velocity shifts of $\pm 2500\mathrm{km}{\mathrm{s}}^{-1}$ as a visual reference.