The Pre-He White Dwarfs in Eclipsing Binaries. I. WASP 0131+28
Abstract
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 = 10,000 200 K and v1 = 55 10 km s-1, respectively. The combined analysis of the 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 M2 = 0.200 0.002 M, R2 = 0.528 0.003 R, T eff,2 = 11,186 235 K, and L2 = 3.9 0.3 L are in excellent agreement with the evolutionary sequence for a helium-core white dwarf of mass 0.203 M, 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.