Nebular-phase spectrum of SN 2017ati at +148.4 d post-explosion (black line), compared with synthetic spectra from different progenitor models. Upper panel: comparison with Type IIb SN models from Jerkstrand et al. (2015a). The blue line represents a model with a progenitor M_ZAMS = 13 M_⊙, while the red line represents a model with M_ZAMS = 17 M_⊙. Lower panel: comparison with the he7p00 and he8p00 models presented by Dessart et al. (2023). The blue line denotes the he7p00 model with M_preSN = 5.04 M_⊙, while the red line corresponds to the he8p00 model with M_preSN = 5.63 M_⊙. In both panels, the model spectra are scaled to the same distance and the same ⁵⁶Ni mass as SN 2017ati.