Dear colleagues,

ASASSN-25dt was discovered by the ASAS-SN Survey (Ohio State University) on 2025 Sep 05.3 at mag 14.5 V/g. The quiescent counterpart is a 20.1 g mag Pan-STARRS1 object, implying an outburst amplitude of ~5-6 mag.

I have just completed the analysis of my unfiltered CCD observations of ASASSN-25dt, obtained on 2025 Sep 10/11 and Sep 11/12 at CBA Extremadura Observatory. The data were acquired under (mostly) clear skies with a 0.40-m f/5.1 telescope and a Moravian G2-4000 CCD.

The Sep 10/11 light curve showed only irregular, small-amplitude variability. However, the Sep 11/12 light curve clearly exhibits superhump modulations with an amplitude of 0.27 mag. Period analysis with ANOVA, Lomb-Scargle, Generalized-Lomb-Scargle and PDM methods (Peranso 3.1) yields a combined superhump period of 0.0605 +/- 0.0008d. The object was measured at mag CV = 15.9 on Sep 12th.

Image calibration and photometry were carried out with the latest release of Phoranso.

These results establish ASASSN-25dt as a new SU UMa-type dwarf nova. Follow-up observations are encouraged.

I will submit my observations to AAVSO, CBA and VSNET for further analysis.

With kind regards,

Tonny

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Tonny Vanmunster

CBA Belgium Observatory

CBA Extremadura Observatory

http://www.cbabelgium.co http://www.cbabelgium.com/ m

PERANSO : The Light Curve and Period Analysis Software

http://www.peranso.co http://www.peranso.com/ m

PHORANSO: Photometric Reduction and Analysis Software

http://www.phoranso.com http://www.phoranso.com/