The Pre-He White Dwarfs in Eclipsing Binaries. V. TIC 399725538

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, $T_{\rm eff,A}=7194\pm70$ K and $v_{\rm A}\sin i=68\pm9$ km s$^{-1}$. Joint modeling of our RVs and the archival TESS data provided component masses of $M_{\rm A}=1.930\pm0.054$ $M_\odot$ and $M_{\rm B}=0.211\pm0.005$ $M_\odot$, radii of $R_{\rm A}=1.922\pm0.020$ $R_\odot$ and $R_{\rm B}=0.207\pm0.005$ $R_\odot$, and luminosities of $L_{\rm A}=8.87\pm0.39$ $L_\odot$ and $L_{\rm B}=0.546\pm0.034$ $L_\odot$. The surface gravity and distance derived from this modeling are consistent with the model-independent $\log g_{\rm B}$ obtained from the direct observables and with the Gaia distance. The third light of $l_3=0.136\pm0.003$ 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 $f_1=0.0848$ day$^{-1}$. The $f_1$ 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.