HM Sagittae: the symbiotic nova of 1975, fifty years on

A Mira and a white dwarf that erupted in 1975 and never quite came back; its I band fell 1.5 mag in 2021-22 (Goldman et al. 2022) and the archives now hold four more years.

Claims

C1 [computed] Between 2019.0-2021.5 and 2026.0-2026.9 HM Sge faded in every AAVSO band: V by 0.95 mag (12.22 to 13.18, 324 and 250 points), B by 0.90, R by 1.65, I by 0.84 at a random pulsation phase, visual by 0.9.
C2 [computed] The fade grows with wavelength from B and V (0.9 mag) to R (1.65 mag): V-R went from 1.3 (2018-2021 yearly medians) to 0.53 (2026), while B-V stayed at 0.45-0.5. The component that dominates R and I, the Mira, lost more light than the nebular continuum that dominates B and V.
C3 [computed] The V-band changepoint detector puts the onset of the fade at MJD 59972 (2023 Jan 27), with a 0.54 mag step between the medians before and after; the yearly medians show the decline running at about 0.2-0.3 mag/yr from 2022 through November 2026 with no sign of flattening.
C4 [computed] In November 2026 HM Sge is 2.5 mag fainter visually than in 1977-1985 (10.6 to 13.1) and fainter than at any time since the 1975 outburst; the 1977-2021 record is a smooth decline to 12.2 with no earlier fade of this kind.
C5 [computed] The Mira pulsation is unchanged by the fade: AAVSO I 2008-2021.5 gives P = 530 d (Lomb-Scargle; 540 d from a sinusoid-plus-trend fit; literature 527-534 d), and the 2022.5-2026.9 I data detrended with a quadratic still pulsate at 540 d with a 0.9 mag amplitude. The I band sits about 1.8 mag below the extrapolation of its 2008-2021 trend (-0.076 mag/yr brightening plus pulsation) by 2025-2026.
C6 [computed] V was 0.24 mag brighter in 2019.0-2021.5 than in 2014-2017 (12.22 vs 12.47): the 2018-2021 high-activity episode of Ikonnikova et al. (2026) is in the AAVSO V record.
C7 [observed] NEOWISE cannot follow this star: all five visits are saturated (W1 about 1.8, W2 about 0.4), so the mid-infrared behaviour through the fade is not measurable from WISE.
C8 [inferred] The 2022-2025 decline to a five-decade minimum is already reported (Ikonnikova et al. 2026, Astron. Rep. 70, 561, arXiv:2603.11777, UBV + JHKLM to 2025; Goldman et al. 2022 RNAAS 6, 159 caught its start in I). An HST proposal (Hack et al. 2025, GO 18051) reads the dimming as a new mass ejection with dust formation beside a jet found in 2021 HST images. What is new here is the record through November 2026, the wavelength dependence of the fade, and the unchanged pulsation.

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