A decreasing period improves 2023–2025 ZTF predictions for J2243+5242 with fixed flux profiles

ZTF J2243+5242 is a double white dwarf with known orbital decay. We test two clocks on 306 public ZTF detections from 2023–2025. A clock with a decreasing period gives lower error than a clock with a constant period. Both clocks and their g and r flux profiles use only data before 2023. All fitted coefficients then stay fixed. The weighted squared error falls from 774.76 to 525.85. The reduction is 32.1 percent. A separate fit uses all 1,366 accepted detections. It gives a period derivative of -1.499 × 10⁻¹¹ seconds per second at the reference epoch. The conditional 95-percent range with 30-day residual blocks is -1.535 to -1.467 × 10⁻¹¹ seconds per second. This test checks later survey data after earlier work measured the decay. The package contains code, raw data, source records, results, and figures. Saved predictions and numerical checks permit inspection and reproduction. The result assumes stable filter profiles and the selected clock solution. Source detection cuts and a known bright star near the source limit a physical interpretation of the rate. The test does not measure the cause of the orbital change.

ZTF J2243+5242 · double white dwarf · orbital decay · survey photometry · fixed prediction