ASASSN-26dt: an SMC Nova with a Luminous Progenitor and CSM
ATel #17878; L. Chomiuk, J, Strader, A. Crossland, R. Kyer, P. Craig (Michigan State), K. Z. Stanek (OSU)
on 6 Jul 2026; 14:10 UT
Credential Certification: Laura Chomiuk (chomiuk@pa.msu.edu)
We highlight a new nova in the Small Magellanic Cloud discovered with ASAS-SN: ASASSN-26dt, also known as AT2026oyp. The source is located at a J2000 position, RA=00:50:45.142, Dec= -71:38:36.96. After peaking at g = 10.5 mag on 2026 June 11, the optical light curve rapidly and smoothly faded with a t_2 = 6 days. In the most recent ASAS-SN photometry from 2026 July 5, the nova now sits at g = 14.0 mag.
As noted on TNS, a redshift of z=0.000426 was measured from a spectrum taken with the SAAO Lesedi 1.0-m telescope on 2026 Jun 27, leading Mlangeni et al. to associate ASASSN-26dt with the SMC. The ASAS-SN light curve implies a peak absolute magnitude M_G = -8.5 mag at an SMC distance of 62 kpc, consistent with the luminosity function of novae (Shafter 2017).
In a spectrum obtained with the SOAR 4.1m telescope on 2026 Jul 5, ASASSN-26dt shows broad H Balmer lines with FWZI = 8200 km/s. Also apparent are lines of both He I and He II, and nebular lines of oxygen and nitrogen. Notably, superimposed on each broad emission line of H and He is a narrow emission component, marginally resolved with FWHM ~ 260 km/s . This narrow emission is likely attributable to circumstellar material in the system, as seen in novae with giant companions (e.g., Munari et al. 2011).
In pre-eruption Gaia DR3 data, ASASSN-26dt is associated with a surprisingly bright, blue star with G = 19.2 mag and BP-RP = 0.17. The Gaia DR3 proper motion of this star is consistent with that of the SMC. At the distance of the SMC and assuming negligible foreground extinction, the absolute magnitude of the progenitor is M_G = 0.2 mag.This is significantly more luminous than cataclysmic variables, including nova-likes, which exhibit absolute magnitudes no brighter than M_G = 3 mag (e.g., Abrahams et al. 2022). A quiescent M_G = 0.2 mag is a bit less luminous than typical for Be stars, which have absolute magnitudes M_G = -0.3 mag or brighter (Radley et al. 2025, Santamaray et al. 2025). A preliminary fit to the SED of pre-eruption photometry across a wide wavelength range (g,r,i,Y,J,K) finds a reasonable fit to a star with Teff ~ 10800K at the SMCâs metallicity. However, the star is more luminous than expected for a main sequence star of this Teff, suggesting that it could be somewhat hotter with some circumstellar extinction, or that there is an additional warm source of emission in addition to the stellar photosphere. A pre-outburst B-band light curve from the EROS survey shows irregular large amplitude variability, including a ~ year long dip of 1.5 mag. There is no known X-ray source associated with this system. Overall, it seems plausible the progenitor star is a less luminous âBe-likeâ star (with spectral type early A), though this is far from conclusive.
Given that this nova possesses an unusual progenitor, we encourage additional follow-up.
