2026ApJ...998..216H

We present a comprehensive photometric study of the eclipsing dwarf nova OY Car based on ground-based observations and archival data from the Transiting Exoplanet Survey Satellite (TESS) and AAVSO. Eclipse timing analysis reveals a secular increase in the orbital period at a rate of +5.70 × 10^–13 s s^–1, superimposed with a cyclic variation with an amplitude of 57.11 s and a period of 33.41 yr. This period increase cannot be attributed to either mass transfer or a period bouncer, suggesting the presence of an unrecognized mechanism. The cyclic oscillation is most effectively interpreted as a gravitational perturbation from an undetected circumbinary object. The potential tertiary companion is estimated to be a giant planet with a mass of 11.5(±3.1)M_Jup in a circular orbit. The TESS light curve captures one superoutburst and one normal outburst. Analysis of the eclipse depth versus the out-of-eclipse flux reveals a linear correlation during quiescence, but with a lower slope before the superoutburst, consistent with disk structure changes predicted by the thermal-tidal instability model. Deviations from linearity during outbursts manifest as hysteresis loops, indicative of significant disk expansion. Eclipse analysis reveals that the normal outburst is of the outside-in type, while the superoutburst exhibits a quasi-periodic O - C modulation indicating a precessing eccentric disk. We derive a mean superhump period of 0.06456 day via the beat relation, which yields a mass ratio of q = 0.1049 using an empirical relation. We also study the superhump evolution, identifying the standard three-stage evolution. The positive period derivative in stage B and the discrepancy between dynamical and empirical mass ratio estimates confirm the significant influence of pressure effects on the accretion disk precession.