LY UMa appeared bluer in the brighter of the two observing runs with both B and V measurements, according to a preprint that estimates its mass donor as a K7-K8V star. The authors link the color pattern to a possible contribution from an accretion flow, but the photometry does not establish that interpretation or resolve the system's classification.
The preprint's front matter records submission on 7 January 2026 and acceptance on 1 August 2026.
The brightness shift
Across the three listed V-band observations, LY UMa was recorded at 15.26 plus or minus 0.03 magnitude on 14 April 2024, 15.43 plus or minus 0.02 on 28 April 2025 and 15.57 plus or minus 0.02 on 2 June 2025. The reported V magnitude rose from the brighter earlier reading to the fainter later one.
The runs with both filters yielded B=16.27, V=15.26 and B-V=1.01 on 13/14 April 2024, compared with B=16.57, V=15.43 and B-V=1.14 on 28 April 2025. The paper describes the star as bluer when brighter and links the pattern to an accretion-flow contribution to optical brightness. The measurements show a pattern, but they do not establish that an accretion flow caused it.
The observations were obtained with the 2.0-meter and 50/70-centimeter Rozhen telescopes and the 60-centimeter Shumen telescope. The researchers processed the data with IRAF and calibrated the photometry against comparison stars numbered 4, 5, 7 and 8, using their Gaia magnitudes as the reference.
Reading the light from the donor
To turn the apparent readings into a distance-adjusted brightness estimate, the authors used a distance of 310 plus or minus 2 parsecs and assumed there was no interstellar extinction toward LY UMa. They estimated its absolute V magnitude, the brightness after that distance adjustment, at 7.94 to 8.10 during the observations. The analysis did not report uncertainty propagation for that derived range.
The proposed donor type depends on how much of the system's V-band light is assigned to the K star. Under an assumed contribution of 70% to 95%, the authors estimated the star's absolute V magnitude at 8.49 to 8.00. Their conclusion identifies the mass donor as K7-K8V, with an absolute V magnitude of 8.0 to 8.5.
That estimate does not resolve LY UMa's classification. The discussion says prior work considered a dwarf-nova classification most probable, although it did not completely explain the observed characteristics. The report presents the photometry as consistent with an optical accretion-flow contribution, rather than as a definitive classification.
A second object in the field
The same observing field contained several quasars and galaxies, with Mrk 153 identified as the brightest. About 7 arcminutes from LY UMa, it was estimated at V=14.81 plus or minus 0.03 on 28 April 2025 at 21:51; the galaxy's nucleus was described as approximately 0.11 magnitude fainter. This is a single estimate for a potentially variable nucleus, so it does not establish that Mrk 153 changes over time.
What remains unresolved
The donor result remains model-dependent because it uses an assumed K-star share of the V-band light. The absolute-magnitude estimate depends on the adopted distance and no-extinction assumption, and the report does not give uncertainty propagation for the derived magnitudes or a formal uncertainty for the spectral classification.
Several questions remain open: whether LY UMa is definitively a dwarf nova, how much of its light comes from the donor and the accretion flow, and whether Mrk 153 varies measurably. The report points to spectroscopy and denser time-series photometry for testing the donor classification and accretion-flow interpretation, while long-term monitoring could assess dwarf-nova behavior, outbursts, orbital variability and changes in Mrk 153.
The work forms part of project KP-06-H98/8 on accretion flows in binary stars, funded by the Bulgarian National Science Fund. Its acknowledgments also cite Bulgaria's National Roadmap for Scientific Infrastructure and support for D. Marchev from the Science Fund of Shumen University.
Paper data and sources
Original title: BV photometry of the cataclysmic variable LY UMa
Authors: L. Dankova, D. Marchev, S. Peneva, R. Zamanov
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
Original paper · Full text