Long-term Orbital Period Variations of the Eclipsing Dwarf Nova HT Cas

Abstract

We present a comprehensive analysis of the long-term orbital period variations in the short-period eclipsing dwarf nova HT Cas. By combining our new high-precision mid-eclipse times obtained between 2015 and 2026 with archival data, we constructed an updated $O-C$ diagram spanning a $\sim$48-years. Statistical analysis confirms outbursts do not cause systematic phase shifts, validating the use of all activity states. Through MCMC modeling, we show that the $O-C$ variations require a two-companion configuration. A free-eccentricity LTT model captures the variations but yields unconstrained posteriors and a highly eccentric outer orbit ($e_3 \sim 0.94$) that instantly collapses in N-body dynamical simulations. Imposing a circular constraint ($e=0$) resolves these mathematical degeneracies, yielding well-constrained posterior distributions. This dynamically stable model identifies two hypothetical circumbinary companions with minimum masses of $\sim 9.8 M_{Jup}$ and $\sim 5.0 M_{Jup}$, and periods of $\sim 32.6$ and $\sim 15.1$ years. Besides, this configuration inherently produces a negative quadratic term ($Q = -1.23 \times 10^{-14}$ days), aligning with secular period decrease predicted by standard CV evolution theory below the period gap. Refined energy-budget tests reveal that classical Applegate mechanisms require significantly more energy than the secondary star provides, indicating they cannot independently drive the modulations. While advanced magnetic frameworks may offer theoretical alternatives, our findings demonstrate that a dynamically stable two-companion architecture provides a highly robust and physically viable explanation, consistent with second-generation planet formation within a post-common-envelope disk.