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Description
We report the first BV light curves and high-resolution spectra of the post-mass transfer binary star WASP0131+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 Teff_1_=10000{+/-]200K and v_1_sin_i_=55{+/-}10km/s, 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{+/-}0.002M_{sun}_, R_2_=0.528{+/-}0.003R_{sun}_, Teff_2_=11186{+/-}235K, and L_2_=3.9{+/-}0.3L_{sun}_ are in excellent agreement with the evolutionary sequence for a helium-core white dwarf of mass 0.203M_{sun}, and indicates that this star is halfway through the constant luminosity phase. The results presented in this article demonstrate that WASP0131+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.
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