Dear colleagues,
The new dwarf nova AT 2025tzw (TCP J21092606+3548416) is now in superoutburst. Optical transient (15.6 in clear band) was detected at 2025-08-15 12:34:33.600 UT by Yuji Nakamura et al. ASAS-SN photometric survey detects AT 2025tzw at 2025-08-15.236 UT with g magnitude 15.866, and detects no transient 2025-08-14.245 UT with limit 16.594. ZTF shows quiescent state magnitude of about 21.5 and three missed historical superoutbursts in September 2023, April 2021 and June 2018, peaking at 16.2-16.4m in zg, which allows us to assume supercycle duration of about 3 years and superoutbursts amplitude of about 5.1-5.3m. The absence of normal outbursts makes the object's status ambiguous between the classical SU UMa and WZ Sge subgroups.
Dr. N. P. Ikonnikova and Dr. S. Yu. Shugarov report presence of superhumps with period ~0.075 days and amplitude about 0.24m based on observations with 60cm Ritchey–Chrétien telescope at 2025-08-16. This may indicate the absence of the early superhumps phase. Color indices are U-B = -0.6 B-V = 0.12 V-R = 0.04 R-I = 0.1.
R. Ottaviani and P. Ochner report spectroscopic classification of AT 2025tzw as CV star with a blue continuum with Balmer absorption except for H alpha. Classification report is available at https://www.wis-tns.org/object/2025tzw
Monitoring photometric observations are encouraged.
Sincerely, Alexander Tarasenkov