IY Lyr: A Thick-disk First-overtone RR Lyrae Star with a Possible Neutron Star Companion

IY Lyr, historically misclassified as an eclipsing binary, has been previously identified as a first-overtone RR Lyrae star (RRc star). Using multiband photometry (All-Sky Automated Survey for Supernovae, Zwicky Transient Facility, TESS, and our BVRI data), Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, and Gaia astrometry, we investigate its pulsation, binarity, and Galactic population. From O−C analysis, we detect a long-term period decrease and a light-travel time effect with an orbital period of 3.94 ± 0.09 yr, eccentricity of 0.46 ± 0.15, and a mass function of 0.65 ± 0.14 M ⊙ . The companion is independently supported by radial velocity residuals and Gaia proper motions. Combined constraints yield an orbital inclination of 94 . ° 2 ± 1 . ° 1 and a companion mass of 1.37 ± 0.19 M ⊙ . Chemical abundances ([Fe/H] ≃ −1.0 ± 0.1, [ α /Fe] ≃ +0.27 ± 0.03) and dynamics ( L z ≃ 1287 ± 35 kpc km s −1 , Z max ≃ 1.17 ± 0.10 kpc) identify IY Lyr as likely an old, high- α , thick-disk star. The companion mass lies at the peak of the neutron star mass distribution, and the system’s age excludes a main-sequence star; we conclude the companion is most likely a typical neutron star, although a massive white dwarf near the Chandrasekhar limit cannot be ruled out. IY Lyr is among the few RRc binaries with a compact companion supported by multiple methods, and it has important implications for thick-disk binary evolution and neutron star formation.