Photometric and Spectral Evolution of the Symbiotic Nova HM Sagittae Since 2003

We present photometric and spectral monitoring of the symbiotic system HM Sge, which consists of a Mira variable with a dust shell and a hot white dwarf ionizing the surrounding gaseous nebula. The system underwent a nova-like outburst in 1975 and experienced a high-activity episode during 2018–2021. $$UBV$$ photometry from 2003 to 2025 shows monotonic fading at $$ \sim {\kern 1pt} {{0.05}^{m}}$$ /yr until 2018, followed by a $${{0.3}^{m}}$$ brightening peaking in 2021 and a decline by 2025 to the faintest level in five decades of monitoring. Near-infrared observations ( $$JHKLM$$ , 2009–2025), combined with archival data, reveal Mira pulsations with a period of 532d and long-term variability driven by changes in the optical depth of the dust shell. Spectral monitoring (2016–2025) reveals a substantial evolution in the emission spectrum of the gaseous envelope. The 2018–2021 high-activity episode was accompanied by enhanced fluxes in recombination lines (H I, He I, He II) and forbidden transitions ([O I], [Ar V], [Fe VI], [Ca VII]), together with a 17-fold brightening of the Raman-scattered O VI $$\lambda $$ 6725 Å line. We report the first detection of the [Fe X] $$\lambda $$ 6374 Å line in HM Sge. Monitoring this line from 2007 to 2025 shows its equivalent width growing through 2017—indicating gradual coronal heating—then declining by 2021, likely reflecting altered accretion conditions and/or hot-component properties during the high-activity episode. We propose that both the 1975 outburst and the 2018–2021 high-activity episode may be linked to periastron passage of the binary components; if so, the $$ \sim {\kern 1pt} 46$$ -year interval would constrain the system’s orbital period.