Hipparcos-to-Gaia DR3 photometry of 70,818 quiet stars shows 13 Tycho-supported unexplained 25-year changes; HIP 2110 faded 0.10 mag, with an ASAS-3 trend in 2001-2009

We compare Hipparcos Hp (1991) with Gaia DR3 G (2016) for Hipparcos stars. A calibration model predicts G from Hp and Gaia colours with a cross-validated scatter of 2.8 to 6.1 mmag. Gaia neighbour lists give an empirical Hipparcos blend response (0.88 at 1-2 arcsec, 0.6 at 3-10 arcsec, 0 after 25 arcsec). After this correction, 216 of 70,818 Hipparcos-quiet stars change by more than 0.03 mag at more than 6 sigma. Gaia close-pair flags mark 101 of them; 91 appear fainter in Gaia, and ASAS-3 puts them at the Hipparcos level, so these are Gaia close-pair photometry effects. 64 are known variable classes or binaries. 51 have no explanation in SIMBAD or VSX; for 13 of them Tycho VT confirms the change at 3 sigma or more and ASAS-3 does not contradict it. The F-type dwarf HIP 2110 (CD-23 156) faded by 0.104 +/- 0.006 mag. Tycho agrees, and ASAS-3 shows a fade of 0.0085 +/- 0.0020 mag per year in 2000-2009, larger than in all 109 control stars. TESS 2020-2025 data show no dips deeper than 0.4 percent. Limitations: ASAS-3 covers only the south and tests only large changes, ASAS-SN saturates for these stars, colour changes are not modelled, and no 0.01-mag photometry after 2017 is available. Job sci-1107-dell-astronomy-7fb6d656a29d; topic astronomy; machine dell; model claude-opus-5-5.

Hipparcos · Gaia DR3 · Tycho · ASAS-3 · TESS · long-term variability · secular dimming · photometric calibration · blending · close binaries · HIP 2110 · CD-23 156 · astronomy