The light curve of T Cephei: humps, long-term changes and recent phenomena
The long-period Mira variable T Cephei has been observed for nearly 150 years, during which more than 130 cycles have been recorded. Using visual observations from the AAVSO database, maxima, minima, pulsation periods, amplitudes and hump characteristics were estimated for 136 cycles from the late nineteenth century to 2026. Most cycles display a temporary slowing of the rise to maximum light ("humps"). These humps show significant cycle-to-cycle persistence in occurrence, duration and brightness, and are more likely to occur during high-amplitude cycles. Hump onset magnitudes exhibit a lower limit near V ~ 9.3, with many humps beginning near V ~ 8. Cycles with larger amplitudes/deeper minima tend to have humps which are dimmer and more gradual. Comparison with published spectroscopy supports the interpretation that the humps are associated with changes in TiO-dominated molecular opacity within the stellar atmosphere. A qualitative model involving secondary pressure waves traversing TiO-rich layers is proposed to account for the observed hump morphology and its variability. The most recent cycles of T Cep have shown unprecedented behaviour. Pulsation amplitudes have fallen to historically low values, the maximum-to-maximum period has shortened substantially, and the light curve has become increasingly asymmetric. In Cycle 139, a substantial hump appeared on the descending branch, a phenomenon never before observed in T Cep. Similarities with the Mira variable T UMi raise the possibility that these recent changes may be due to evolutionary developments.