MGAB-V233: superhumps detected during the current superoutburst

ATel #18056; D. Yu. Rusanova (ASPD TShNU of Kyiv, Ukraine), A. O. Simon (ASPD TShNU of Kyiv, Ukraine), A. Yu. Korosko (ASPD TShNU of Kyiv, Ukraine), P. J. Mikolajczyk (U. Wroclaw/NCBJ, Poland), L. Wyrzykowski (U. Warsaw/NCBJ, PL/EASST. eu, Poland), K. Kotysz (U. Wroclaw/U. Warsaw, Poland), J. K.T. Qvam (Horten Videregaende Skole, Norway), V. Bozza (U. Salerno, Italy), S. Dufoer (VVS, Belgium), S. Kurowski (Jagiellonian University, PL/SOA Bolecina, Poland), V. V. Vasylenko (ASPD TShNU of Kyiv, Ukraine)
on 18 Sep 2026; 21:05 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Andrew Simon (skazhenijandrew@gmail.com)

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MGAB-V233 is a well-known ER UMa-type dwarf nova characterized by frequent superoutbursts and a short supercycle of approximately 70-90 days. Its superoutbursts typically last about 20-30 days. Between consecutive superoutbursts, typically six to nine normal outbursts are observed, consistent with the high outburst activity characteristic of ER UMa-type systems.

Based on previous superoutburst activity, our Outbursts Alerting System (OASys) predicted the next superoutburst to occur within MJD 61292-61313 (Fig. 1). A new outburst subsequently detected in the ATLAS light curve began within the predicted interval. Following its detection, we immediately initiated time-resolved photometric observations within the BHTOM community to search for superhumps and confirm the nature of the event.

Observations were carried out at five facilities: the 0.7-m f/4 AZT-8 telescope at the Observational Station Lisnyky of Taras Shevchenko National University of Kyiv, the 0.68-m telescope at the Horten Local Observatory, the 0.6-m Telescope of the University of Salerno Astronomical Observatory, the 0.4-m Telescope of the EEye Astronomical Complex and the 0.5-m OAUJ Cassegrain Telescope.

Time-series photometry obtained over three nights was analyzed jointly to improve the accuracy of the superhump period determination. The combined data clearly reveal superhumps with a period of 0.09565(6)^d (Fig. 2) and a peak-to-peak amplitude of approximately 0.3^m (Fig. 3). Given the measured superhump period of 0.09565^d, the orbital period is expected to lie within the period gap, close to its lower boundary. All materials related to this observation are available on A.O. Simon's personal website.

The detection of superhumps is consistent with the known ER UMa nature of MGAB-V233 and confirms the superoutburst nature of the current event.

Further observations are strongly encouraged. Observers are invited to upload their observations to the object's BHTOM page, where all submitted data are automatically processed: https://bhtom.space/public/targets/MGAB-V233.

BHTOM is based on the open-source TOM Toolkit by LCO and has been developed with funding from the OPTICON-RadioNet Pilot (ORP) of the European Union's Horizon 2020 research and innovation programme under grant agreement No 101004719 (2021-2025). This project has received funding from the European Union's Horizon Europe Research and Innovation programme ACME under grant agreement No 101131928 (2024-2028).