Stage-dependent superhump waveform evolution and non-stationary positive-superhump timing in the near-period-gap dwarf nova YZ Cancri

RESUMO

We present a photometric and timing study of the near-period-gap SU~UMa-type dwarf nova YZ~Cancri, based on nearly continuous Transiting Exoplanet Survey Satellite (TESS) photometry and long-baseline ground-based observations. Our main observational result is that the positive-superhump (SH) waveform follows a repeatable, stage-dependent sequence during superoutbursts (SOs). In the two well-covered TESS SOs, and consistently in the long-baseline ground-based SO sample, the plateau waveform evolves from an early saw-tooth profile (ST), through a double-humped profile with unequal maxima (DHd), to a more symmetric double-humped profile with nearly similar maxima (DHs). The global and time-resolved Lomb--Scargle periodograms show that power near the positive-SH time scale and its harmonics is concentrated during SOs, whereas quiescent and normal-outburst intervals lack a comparably persistent SH-band signal. The dense TESS maxima--minima timing sequence shows different clock stability in different waveform stages: precursor modulations have slightly longer local periods, the middle-to-late DHs plateau is the most regular timing interval, and the post-plateau evolution is affected by phase switching and possible secondary/late-SH contamination. The TESS data also reveal profile-clock coupling, with SH amplitude and rise/decay durations evolving together with the timing residuals. As a secondary timing constraint, we obtain a common TESS-timing-based mean positive-SH period of $P'_{\rm sh}=0.09043(27)$~d, corresponding to a SH excess of $4.03(31)\%$ and an approximate mass ratio of $q=0.175(11)$. The repeatable ST--DHd--DHs sequence makes YZ~Cnc a useful system near the lower edge of the period gap, and may trace the growth, redistribution, stabilization, and decay of the light source associated with an eccentric, precessing accretion disk during SOs.