Spectral Evolution Evolution of the Very Fast Nova Sagittae 2026

Abstract Classical novae are thermonuclear explosions on the surface of white dwarfs (WDs) accreting mass from less evolved stellar companions. The photometric and spectral properties of novae are mostly determined by the WD mass, with more massive progenitors producing faster novae with He and N emission lines brighter than Fe ii . Recent evidence shows, however, the spectral class to evolve following the sequence: early He/N → Fe ii → late He/N. We apply here our quantitative method to assess the spectral class and time evolution of the very fast Nova Sagittae 2026, finding that it evolved from Fe ii at peak to late He/N (in contrast with its speed class) with no detected early He/N phase.