2026AJ....171..181L

We present echelle spectra of TIC 399725538 obtained in Korea and Thailand to investigate the physical properties and evolutionary scenarios of EL CVn-type binaries. The time-series spectra yielded the radial velocities (RVs) of the primary component and its atmospheric parameters, T_eff,A = 7194 ± 70 K and ${v}_{{\rm{A}}}\sin i=68\pm 9$ km s^–1. Joint modeling of our RVs and the archival TESS data provided component masses of M_A = 1.930 ± 0.054 M_⊙and M_B = 0.211 ± 0.005 M_⊙, radii of R_A = 1.922 ± 0.020 R_⊙and R_B = 0.207 ± 0.005 R_⊙, and luminosities of L_A = 8.87 ± 0.39 L_⊙and L_B = 0.546 ± 0.034 L_⊙. The surface gravity and distance derived from this modeling are consistent with the model-independent $\mathrm{log}{g}_{{\rm{B}}}$ obtained from the direct observables and with the Gaia distance. The third light of l₃ = 0.136 ± 0.003 comes mainly from two neighboring stars, TIC 399725539 and TIC 399725544. Comparison with stellar models indicates that TIC 399725538 A lies within the instability region of δ Sct-γ Dor hybrids, whereas its extremely low-mass companion is markedly underluminous compared to theoretical white-dwarf (WD) counterparts. Multifrequency analysis of the binary-subtracted residual lights revealed three significant signals, two of which correspond to aliases at two and four times the dominant frequency f₁ = 0.0848 day^–1. The f₁ frequency is likely a γ Dor-type pulsation arising from the brighter A component, though further confirmation is required. Our results demonstrate that TIC 399725538 is a short-period EL CVn system belonging to the thick-disk population, consisting of a main-sequence γ Dor pulsator and a helium-core WD precursor formed through stable mass transfer.