Abstract
U Sagittarii is a Classical Cepheid variable star located in the Messier 25 cluster. 11 observations were made in Cousins V and I bands from 2025-10-14 to 2025-11-15 using the Slooh Chile 2 Telescope. Photometric comparison stars were selected based on angular separation, SNR, and SIMBAD magnitude.
Sinusoidal light curves with the established period of 6.74 days were fit for Cousins V and I band data, with R2 values of 0.9301 and 0.7034 respectively. A strong correlation (r = 0.8450) was found between Vc - Ic color index and Vc magnitude, supporting the existence of the Kappa-mechanism.
Distance calculation using Leavitt's law were accurate for Ic but inaccurate for Vc (δ = 40.77%). Future studies can further explore the Kappa-mechanism after adjusting for dust extinction.
Radial Velocity & Radius derived from Hocdé et al. (2026, arXiv:2603.11748)
The simulation above now includes a reconstructed radial velocity (RV) curve derived using the method of Hocdé et al. (2026). That paper calibrates tight empirical relations between the Fourier parameters of the V-band light curve and the Fourier parameters of the RV curve, using a sample of 81 short-period Galactic Cepheids. The reconstruction requires only the pulsation period and the LC amplitude ratios R21 and R31.
For U Sgr, a 7th-order Fourier decomposition of the Berdnikov light curve yields R21(LC) = 0.363 and R31(LC) = 0.134. Applying Table 1 of Hocdé et al., the RV semi-amplitude is A1(RV) = 14.7 km/s, and the full reconstructed curve has a peak-to-peak amplitude of ~40 km/s. A phase lag of Δφ1 = −0.28 rad (Ogłoza et al. 2000) is applied to align the RV curve with the light curve, so the RV maximum (rapid infall) occurs just before photometric maximum.
The radius variation (green strip) is derived by integrating the pulsational component of the RV curve via the Baade-Wesselink method, using a projection factor p = 1.27 (Mérand et al. 2005, CHARA direct measurement of δ Cep; consistent with the Trahin et al. 2021 mean of p = 1.26 ± 0.07 across 63 Galactic Cepheids). The mean radius anchor uses the Trahin et al. (2021) P-R relation calibrated with Gaia EDR3 parallaxes: log R = 1.763 + 0.653(log P − 0.9), giving Rmean = 52.1 R☉. This is consistent with interferometric measurements of comparable Cepheids (δ Cep at P = 5.37 d has R ≈ 43 R☉ from interferometry; the Trahin relation gives 44.8 R☉). The BW integration yields a peak-to-peak radius variation of ΔR ≈ 5.4 R☉ (range 48.7–54.1 R☉), with the shape being independent of the anchor.