The pre-He white dwarfs in eclipsing binaries - III. WASP 1625-04

Abstract

1SWASP J162545.15-043027.9 (WASP 1625-04) has been announced as one of EL CVn candidates showing total primary eclipses and ellipsoidal variations. This paper presents the absolute properties of the binary star, based on our high-resolution spectroscopy conducted from 2015 through 2020. From the spectral analysis, the radial velocities (RVs) for both components were obtained with the effective temperature T_{eff, 1} = 8990 ± 200 K and the rotational rate $v_1\sin \, i=53\pm 5$ km s^-1 for the more massive primary. The RV measurements were analysed with archival WASP photometry. From the modelling we obtained: M₁ = 1.745 ± 0.013 M_⊙, M₂ = 0.187 ± 0.002 M_⊙, R₁ = 1.626 ± 0.008 M_⊙, R₂ = 0.290 ± 0.003 M_⊙, L₁ = 15.5 ± 1.4 L_⊙, and L₂ = 1.84 ± 0.16 L_⊙. In the Hertzsprung-Russell diagram, WASP 1625-04 A lies on the zero-age main sequence and its companion accords well with the helium-core white dwarf models of 0.19 M_⊙ in the constant luminosity phase. Our improved results demonstrate that WASP 1625-04 is a typical EL CVn-type binary with a low-mass ratio and M₂ combination in the thin-disc population and is the product of the stable, non-conservative mass transfer of the precursor binary.