The Pre-He White Dwarfs in Eclipsing Binaries. III. WASP 1625-04

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 $T_{\rm eff,1} = 8990 \pm 200$ K and the rotational rate $v_1\sin$$i=53\pm5$ km s$^{-1}$ for the more massive primary. The RV measurements were analyzed with archival WASP photometry. From the modelling we obtained: $M_1 = 1.745 \pm 0.013$ M$_\odot$, $M_2 = 0.187 \pm 0.002$ M$_\odot$, $R_1 = 1.626 \pm 0.008$ M$_\odot$, $R_2 = 0.290 \pm 0.003$ M$_\odot$, $L_1 = 15.5 \pm 1.4$ L$_\odot$, and $L_2 = 1.84 \pm 0.16$ L$_\odot$. 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$_\odot$ 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 $M_2$ combination in the thin-disk population and is the product of the stable, non-conservative mass transfer of the precursor binary.