A Deep Precursor Dip-Main Superoutburst Sequence in VW Hydri Observed with TESS: High-cadence Constraints on the Thermal-Tidal Instability Model

We present 120 s cadence TESS observations of three superoutbursts of the SU UMa-type dwarf nova VW Hydri. Two events (SO2 in Sectors 87+88 and SO3 in Sector 93) exhibit a temporally pronounced precursor dip followed by a rapid rise into the main superoutburst plateau. This morphology, previously seen in Kepler light curves of V1504 Cyg and V344 Lyr, is a key prediction of the thermal-tidal instability (TTI) model when a normal (precursor) outburst expands the disk only marginally beyond the 3:1 resonance radius, allowing the tidal instability to grow slowly and produce a deep dip approaching quiescence before rapid amplification drives the main superoutburst. A sliding-window time-frequency analysis reveals superhump power already during the decline and near minimum light, with a smooth period evolution across the dip and stabilization after the system returns to the hot state, consistent with the growth and saturation of disk eccentricity at the 3:1 resonance. From the stabilized Stage A superhump periods, we infer a representative mass ratio q = 0.131 ± 0.002. Combined with either a typical SU UMa white-dwarf mass prior or the semiempirical donor sequence at an orbital period of 107 minutes, the implied component masses are M₁ ≃ 0.6-1.0 M_⊙ and M₂ ≃ 0.08-0.14 M_⊙, ruling out a brown dwarf donor and establishing VW Hyi as a benchmark system for testing tidal instability models in low-q dwarf novae.