The Pre-He White Dwarfs in Eclipsing Binaries. I. WASP 0131+28

We report the first BV light curves and high-resolution spectra of the post-mass transfer binary star WASP 0131+28 to study the absolute properties of extremely low-mass white dwarfs. From the observed spectra, the double-lined radial velocities were derived, and the effective temperature and rotational velocity of the brighter, more massive primary were found to be $T_{eff,1}=10,000\pm 200$ K and $v_1\sin i$ = 55 ± 10 km s⁻¹, respectively. The combined analysis of the Transiting Exoplanet Survey Satellite (TESS) archive data and ours yielded the accurate fundamental parameters of the program target. The masses were derived to about 1.0% accuracy and the radii to 0.6%, or better. The secondary component's parameters of $M_2=0.200\pm 0.002$ $M_{\odot }$, $R_2=0.528\pm 0.003$ $R_{\odot }$, $T_{eff,2}$ = 11,186 ± 235 K, and $L_2=3.9\pm 0.3$ $L_{\odot }$ are in excellent agreement with the evolutionary sequence for a helium-core white dwarf of mass 0.203 $M_{\odot }$, and indicates that this star is halfway through the constant luminosity phase. The results presented in this article demonstrate that WASP 0131+28 is an EL CVn eclipsing binary in a thin disk, which is formed from the stable Roche-lobe overflow channel and composed of a main-sequence dwarf with a spectral type A0 and a pre-He white dwarf.