TESS Pulsation Timing Reveals the JWST Calibration Star HD 158485 as a Double-pulsator Binary

Abstract HD 158485 is an A-type δ Scuti star in the CALSPEC network and the JWST absolute-flux calibration program. Gaia DR3 assigns it a binary classification probability close to unity but provides no orbital solution. Using TESS observations from Sectors 14–26, 40–41, 47–60, and 73–86, we measure the pulsation phases of six δ Scuti modes. The frequencies form two groups with opposite light-travel-time delays, revealing two pulsating components. A direct light-curve forward model yields an orbital period of P orb = 382.8 0 − 0.76 + 0.80 days, eccentricity e = 0.24 2 − 0.046 + 0.045 , projected light-time amplitudes of 195 . 8 − 8.9 + 9.2 and 239 . 7 − 6.8 + 6.8 s, and a dynamical mass ratio of q = M B / M A = 0.81 8 − 0.045 + 0.046 . A blind prediction based only on the first two long TESS campaigns reproduces the third campaign without updating the orbital parameters, with a Pearson correlation coefficient of r = 0.983. Its held-out rms is 83.8 s, compared with 289.9 s when the period is fixed at 365.25 days. A 5.15 σ Hipparcos–Gaia EDR3 proper-motion anomaly independently indicates unresolved astrometric motion. The unresolved companion can also affect spectrophotometric predictions: a conditional double-pulsator-binary (PB2)-constrained two-atmosphere model differs from the corresponding single-star atmosphere prediction by 1.6% over 1–5 μ m and 2.5% over 5–28 μ m. HD 158485 therefore shows that a calibration-relevant binary can lack a catalogued orbital solution while two independent pulsation clocks reveal and measure its orbit.