Kavli Affiliate: Avi Shporer
| Summary:
We present 120s 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 pronounced, 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 $q = 0.131 pm 0.002$. Combined with either a typical SU UMa white-dwarf mass prior or the semi-empirical donor sequence at an orbital period of 107~min, the implied component masses are $M_1 simeq 0.6$–$1.0,M_odot$ and $M_2 simeq 0.08$–$0.14,M_odot$, ruling out a brown-dwarf donor and establishing VW~Hyi as a benchmark system for testing tidal-instability models in low-$q$ dwarf novae.
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