## 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. All times are converted to BJD in the TDB standard.

## 2. Timing model

We fit a constant-period model and a model with a steadily changing period. The changing-period model is preferred, with a period derivative of -29.4 ± 1.3 ms per year.

## 3. Tidal quality factor

For the stellar mass and radius of WASP-12, the period derivative implies a modified tidal quality factor of about 1.75 × 10^5. This is at the low end of values expected for a main-sequence host, which suggests the star may have begun to evolve off the main sequence.

## 4. Conclusion

The orbit of WASP-12b is decaying, and the decay is fast enough to be measured again with a few more years of transits.