CN Trianguli: A W-type Contact Binary Exhibiting A-type Light Curve via Starspot-induced Dimming

Abstract We present comprehensive photometric observations and analysis of the totally eclipsing binary CN Tri. The photometric solutions suggest it as an extremely low mass ratio contact binary system with q ∼ 0.08. Multiband photometry obtained using the WH50 and XL85 telescopes, along with high-cadence time-series photometric data from the Transiting Exoplanet Survey Satellite (TESS), reveals that CN Tri is fundamentally a W-type contact binary. However, the light curve morphology from TESS observations exhibits characteristics typical of A-type systems. We successfully reconciled this apparent contradiction through modeling a time-evolving starspot near orbital phase 0.5, which provides a comprehensive explanation for the anomaly observation in this system. This case study demonstrates that starspot-induced light curve distortions can lead to misclassification of contact binary types, offering a valuable framework for interpreting similar anomalies in other W UMa systems with discrepant photometric classifications.