## 1. Data

We collected 268 transit mid-times of WASP-12b from the literature, TESS sectors 20, 43, 44, 45 and 71, and our own photometry from 2023, and 30 occultation times. All times are converted to BJD in the TDB standard.

## 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.

## 3. Results

Orbital decay is preferred over precession. The period derivative is -29.0 ± 1.1 ms per year.

## 4. Tidal quality factor

For the stellar mass and radius of WASP-12, the period derivative implies a modified tidal quality factor of about 1.7 × 10^5.