## 1. Data

We collected 318 transit mid-times of WASP-12b from the literature, TESS sectors 20, 43, 44, 45, 71 and 72, and our own photometry from 2023 to 2025, and 41 occultation times. All times are converted to BJD in the TDB standard, and each literature time is refit from its light curve where the light curve is public.

## 2. Timing model

We compare three models by nested sampling: a constant period, a steadily changing period, and apsidal precession of a slightly eccentric orbit. Occultation times break the degeneracy between decay and precession, because precession shifts transits and occultations in opposite directions. Timing uncertainties are inflated by a jitter term fit jointly with the model.

## 3. Results

Orbital decay is preferred over precession by a Bayes factor above 1000. The period derivative is -29.1 ± 0.9 ms per year, and the residuals show no periodic signal above 30 seconds.

## 4. Tidal quality factor

For the stellar mass and radius of WASP-12 from asteroseismic scaling, the period derivative implies a modified stellar tidal quality factor of (1.7 ± 0.1) × 10^5. This favours a host that has begun to evolve off the main sequence, where tidal dissipation in the radiative core is efficient.

## References

Yee, S. W. et al. (2020). The orbit of WASP-12b is decaying. ApJL 888, L5.