Orbital Modulation Induced by a Third Component in the Overcontact Binary V0554 Dra

Abstract We present the first comprehensive photometric and spectroscopic analysis of the overcontact eclipsing binary V0554 Dra. By simultaneously modeling multiband light curves and radial velocity data using the Wilson–Devinney code, we determine that the system is a W-subtype contact binary with a mass ratio of 3.61 ± 0.01 and a shallow contact degree of 18% ± 2%. The light curves exhibit notable asymmetries near phase 0.5, which are well modeled by introducing a hot spot and a cool spot on the secondary component. The hot spot, located near the inner Lagrangian point, suggests the presence of mass transfer from the less massive primary to the more massive secondary. A detailed ( O − C ) analysis based on 3753 times of light minima reveals a long-term orbital period decrease at a rate of dP / dt = −4.22 × 10 −7 days yr −1 , superimposed with a 1.72 yr cyclic variation. Although mass is transferring from the less massive primary to the more massive secondary, which would typically lead to an increase in the orbital period under conservative conditions, the observed secular decrease suggests that angular momentum loss is the dominant driver of the system’s evolution. Morphological analysis of the Transiting Exoplanet Survey Satellite light curves over a 5.4 yr baseline rules out magnetic activity as the cause of the cyclic variation, supporting the light-travel time effect (LTTE) due to a tertiary component. Assuming LTTE, we estimate the minimum mass of the third body to be 0.68 M ⊙ , with a projected separation of 2.15 au from the binary. Taken together, our findings strongly support the presence of a dynamically significant tertiary companion and reveal V0554 Dra as an exceptionally promising system for studying evolution induced by a third component and potential merger pathways in contact binaries.