Four TESS sectors give a 0.36953162-day binary clock near WASP-178 under a stable harmonic phase model

The known contact binary ASASSN-V J150908.07-424253.6 adds periodic flux to the TESS aperture of WASP-178. We measure its dominant phase in four public 120-second light curves from 2021, 2023, and 2026. After planet-event masks, the sample has 57,439 cadences. A common phase clock gives a binary period of 0.36953162 day with a conditional standard error of 0.00000012 day. This result assumes a stable second-harmonic phase and uses the published source identification. SAP and PDCSAP flux give similar clocks. Changes to the trend, mask, harmonic model, and noise block size give a total period range of 0.0071 seconds. A bounded cycle-count search from 0.3685 to 0.3705 day selects the same clock. The next two tested full-cadence PDCSAP candidates have weighted residual sums larger by 498 and 618. These scores are not false-alarm probabilities. A 0.739-day model gives no consistent gain in held-out predictions. One common four-harmonic waveform, with separate amplitudes, fits worse than separate sector shapes. Its conditional wild block bootstrap tail fractions range from 0.001 to 0.008 across the tested flux series and block sizes. Instrument effects and source blending remain possible causes. The clock is 0.485 seconds longer than the published reference value. That difference does not establish a physical period change. The package preserves the original data, source hashes, code, results, figures, prior-work records, and reproduction instructions.

TESS · WASP-178 · contact binary · ASASSN-V J150908.07-424253.6 · photometric contamination · period measurement · reproducible research