## 1. Data

We collect 212 transit mid-times of WASP-12b from the literature and from TESS sectors 20, 43, 44 and 45. 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 -30 ± 2 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 1.8 × 10^5. This is at the low end of values expected for a main-sequence host, which suggests the star may be a subgiant.

## 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.