BS Cassiopeiae: A Contact Binary with Starspot-Driven Variability and Evidence for Additional Components
Abstract
We present a comprehensive photometric and spectroscopic analysis of the contact eclipsing binary BS Cas, based on 22-yr multi-site light curves and 2-yr high-resolution spectra. The long-term photometry shows pronounced year-to-year changes in the light-curve shape, including eclipse-depth reversals between the primary and secondary minima and variations in the O'Connell effect. The first double-lined radial-velocity (RV) curves of BS Cas are derived from the spectra using broadening-function (BF) profiles. Sharp peaks absent from the 2018 BFs appear in the 2020 BFs, suggesting an additional component along the same line of sight as BS Cas. Simultaneous modeling of the BVR light curves and the RVs shows that BS Cas is a contact binary in which the more massive component is cooler than its less massive companion, with a mass ratio of 2.515 and a fill-out factor of 32.3%. Multi-epoch modeling suggests that the long-term photometric variations arise from evolving starspots and changes in the third-light contribution. The eclipse-timing variation diagram confirms a secular orbital period decrease at a rate of −2.305 × 10−7 days yr−1 consistent with angular momentum loss and mass transfer, along with an 18.376-yr modulation interpreted as a light-travel-time effect from a third body. The TESS timings reveal anti-correlated variations between the primary and secondary minima that appear to result from differential rotation of high-latitude starspots. These results suggest that BS Cas is a physically complex contact binary in which long-term photometric and eclipse-timing variations reflect multiple processes, including magnetic activity and the influence of a third body.
Versions
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BibTeX
@misc{jeong2026cassiopeiae,
title = {{BS Cassiopeiae: A Contact Binary with Starspot-Driven Variability and Evidence for Additional Components}},
author = {Min-Ji Jeong and Chun-Hwey Kim and Kyeongsoo Hong and Mi-Hwa Song and Hye-Young Kim and Jang-Ho Park and Cheongho Han and Joh-Na Yoon},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.30792},
eprint = {2609.30792},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.30792},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - BS Cassiopeiae: A Contact Binary with Starspot-Driven Variability and Evidence for Additional Components
AU - Min-Ji Jeong
AU - Chun-Hwey Kim
AU - Kyeongsoo Hong
AU - Mi-Hwa Song
AU - Hye-Young Kim
AU - Jang-Ho Park
AU - Cheongho Han
AU - Joh-Na Yoon
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.30792
UR - https://arxiv.org/abs/2609.30792
AB - We present a comprehensive photometric and spectroscopic analysis of the contact eclipsing binary BS Cas, based on 22-yr multi-site light curves and 2-yr high-resolution spectra. The long-term photometry shows pronounced year-to-year changes in the light-curve shape, including eclipse-depth reversals between the primary and secondary minima and variations in the O'Connell effect. The first double-lined radial-velocity (RV) curves of BS Cas are derived from the spectra using broadening-function (BF) profiles. Sharp peaks absent from the 2018 BFs appear in the 2020 BFs, suggesting an additional component along the same line of sight as BS Cas. Simultaneous modeling of the BVR light curves and the RVs shows that BS Cas is a contact binary in which the more massive component is cooler than its less massive companion, with a mass ratio of 2.515 and a fill-out factor of 32.3%. Multi-epoch modeling suggests that the long-term photometric variations arise from evolving starspots and changes in the third-light contribution. The eclipse-timing variation diagram confirms a secular orbital period decrease at a rate of −2.305 × 10^−7 days yr^−1 consistent with angular momentum loss and mass transfer, along with an 18.376-yr modulation interpreted as a light-travel-time effect from a third body. The TESS timings reveal anti-correlated variations between the primary and secondary minima that appear to result from differential rotation of high-latitude starspots. These results suggest that BS Cas is a physically complex contact binary in which long-term photometric and eclipse-timing variations reflect multiple processes, including magnetic activity and the influence of a third body.
ER -