A fixed waveform fit to TESS Sector 91 agrees with the known decrease in AR Scorpii spin frequency

We test the spin clock of AR Scorpii in public 20-second TESS Sector 91 data from 2025. We fit spin and orbit parameters together with a Fourier model and separate day baselines. We use 71,167 samples with zero quality flags. The local spin frequency at BJD TDB 2460789 is 737.7009757 cycles/day, with a day-group standard error of 0.0000438 cycles/day. Under a fixed waveform model, comparison with the published 2017 reference gives a mean frequency rate of -5.10e-17 Hz/s. Its approximate 95% interval is -5.53e-17 to -4.67e-17 Hz/s. This agrees with the known decrease in spin frequency without counting cycles across the long data gap. The beat frequency has a larger offset from the published prediction: -0.0001226 cycles/day, with standard error 0.0000389 cycles/day. Both visit halves give less precise beat offsets closer to zero. Data and model controls, an independent linear solve, and synthetic recovery checks are supplied. The intervals assume independent day errors and a locally fixed waveform. A timing or pulse-phase delay that grows by about three seconds across the visit could mimic the measured spin decrease. The result does not establish a new physical torque. The package preserves the original source file, archive provenance, fitted data, covariance, code, checks, figures, and instructions.

AR Scorpii · TESS · white dwarf · spin frequency · beat frequency · time series · reproducible research