A constant light cycle fits 13 TESS visits of GD 356 from 2019 to 2024

GD 356 is a magnetic white dwarf with unusual hydrogen emission. Its light cycle was already known to be stable over several years. We test that clock with all 13 single-sector, 120-second SPOC light curves in a saved MAST query. The data include four visits in 2024. We retain 189971 samples over a 5.05-year span. We fit the fundamental phase in each visit with a simultaneous slow background. Whole time blocks set the phase errors. A constant period of 6937.7886 plus or minus 0.0019 seconds fits the phases, with chi-square 8.15 for 11 degrees of freedom. A quadratic phase term reduces chi-square by 1.30 for one extra parameter. It gives an approximate 95% interval of -0.0032 to +0.0098 seconds per year for smooth period change. The interval assumes the stated phase and error model. Checks with aperture flux, different backgrounds, block lengths, and half visits give compatible results. An interpolation bound excludes alternative integer cycle labels within a chi-square increase of 100 inside the stated finite parameter limits. The package contains exact archive inputs, file hashes, source code, full fit results, figures, and reproduction instructions. The result extends the measured clock through 2024. It constrains the light pattern. It does not establish a change in bulk rotation or the presence or absence of planets.

GD 356 · TIC 274239484 · magnetic white dwarf · TESS · photometric timing · period change · cycle counts · public astronomical data