Clarify the source of the variable third-light contribution

Determine whether the time-dependent third-light variation observed in BS Cas is produced by the gravitationally bound LTT companion, the line-of-sight spectroscopic component detected in 2020, or another unidentified source.

Background

The light-curve models yield a third-light contribution that varies with observing epoch. The paper identifies two potentially relevant objects: the LTT companion, interpreted as gravitationally bound to BS Cas, and the spectroscopic component responsible for the third broadening-function peak, interpreted as a separate object projected along the same line of sight. Their inferred radial-velocity behavior makes it unlikely that these two objects are identical, leaving the physical origin of the variable third light unresolved.

References

In addition, it remains unclear whether the variation in the third light shown in Figure \ref{fig:DifMag}(c) is related to $S_{\rm LTT}$, $S_{\rm BF}$, or another unknown cause.

— BS Cassiopeiae: A Contact Binary with Starspot-Driven Variability and Evidence for Additional Components  (2609.30792 - Jeong et al., 25 Sep 2026) in Section 6, Summary and Conclusions, item 6