Independent Discovery and Photometry of a Possible Very Slowly Evolving Nova in M31

ATel #17576; K. Hornoch, H. Kucakova (Astronomical Institute, Ondrejov, Czech Republic), A. W. Shafter (SDSU)
on 5 Jan 2026; 20:28 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Allen W. Shafter (ashafter@sdsu.edu)

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We report the independent discovery of a new slowly brightening transient object during our survey of the central region of M31. We detected the object for the first time on a co-added 810-s R-band CCD frame taken on 2025 Dec. 11.684 UT with the 0.65-m telescope at Ondrejov. Since our initial discovery, we have continued to follow the object photometrically in order to characterize the lightcurve and determine whether it is a nova or another type of variable star.

The object was independently discovered on Dec. 8.174 UT by D. Perley et al. (ZTF) and designated ZTF25acghjar = AT 2025ajxy. The object has been designated by us as PNV J00432344+4124357 = M31N 2025-12f, and is located at R.A. = 0h43m23s.44, Decl. = +41o24'35".7 (equinox 2000.0), which is 440.5" east and 507.2" north of the center of M31 (see link to a finder chart below).

As shown in the table below containing our photometry, the object brightened very slowly for almost one month reaching 19.0 mag, and will possibly continue to brighten further. Such a gradual, very slow brightening is unusual for a nova (but not impossible, see ATel #15926), but typical of slowly varying Mira variables in M31 that can sometimes masquerade as novae. Given the initial uncertainty surrounding the object, we postponed any discovery announcement until we could reasonably exclude the possibility that the transient is a Mira-type variable star.

Since Mira variables have significantly higher color indices than novae, we took complementary images in I-band to measure (R-I) color of the object (see the table below). On 2026 Jan. 3.8 UT we measured (R-I) = 0.5 ± 0.2 mag which is consistent with a nova, but not with a Mira-type variable.

The following magnitudes were obtained using the 0.65-m telescope at Ondrejov (D65):

 
  Date [UT]    |   Mag | Err  | Filter | Telescope 
 
2025-11-23.682 | >20.4 |      |   R    | D65 
2025-12-11.684 |  19.7 | 0.25 |   R    | D65 
2025-12-24.694 |  19.7 | 0.2  |   R    | D65 
2025-12-26.817 |  19.4 | 0.15 |   R    | D65 
2025-12-28.678 |  20.2 | 0.3  |   R    | D65 
2026-01-03.781 |  19.3 | 0.15 |   R    | D65 
2026-01-03.800 |  18.8 | 0.15 |   I    | D65 
2026-01-04.795 |  19.0 | 0.2  |   R    | D65 
2026-01-04.874 |  19.1 | 0.2  |   R    | D65 
 

We strongly encourage spectroscopic observations to confirm the nova's nature of the object and also follow-up photometric observations to determine the maximum brightness of the object, its timing and also to measure rate of decline (speed class). We note that if the object is a nova, it will exhibit strong Hα emission only after reaching maximum brightness.

Finder chart