AT2025usz is a dwarf nova in its early phase

ATel #17355; M. Dennefeld (IAP & Sorbonne U.), W. Thuillot (Paris Observatory), Th. Midavaine (SAF) and the RAPAS amateur astronomers group
on 23 Aug 2025; 03:34 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Michel Dennefeld (dennefel@iap.fr)

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During a training session at Haute-Provence Observatory (OHP=IAU 511) of amateur astronomers from the RAPAS group, we observed the transient AT2025usz with both the 1.2m telescope in CCD photometry, and the 1.9m telescope in spectroscopy, on August 22, from about UT 23:00 to UT 01:30 the next day. The object is still bright, around 14th magnitude in R band. Two exposures of 20mn each were obtained with MISTRAL at the 1.9m, at a resolution of about 700 and spectral coverage from 420 to 815nm. The object shows a blue continuum, with emission lines from both hydrogen and helium, unresolved at our resolution. No absorption lines are detected, nor PCygni profiles around the emission lines. The only exception is a broad, symmetric absorption around HBeta. These results are in line with those of Isogai et al. (ATel 17347) and suggest that AT2025usz (= ZTF25abjnifk) is a dwarf nova in its early phase, and is therefore worth further observations. We thank J. Balcaen (OHP) and J. French (U. of Edimburgh) for their kind cooperation at the 1.9m telescope.