Pulsations and pre-He white dwarf in the post-mass transfer eclipsing system WASP 1021-28

ABSTRACT We present results from Very Large Telescope/UV-Visual Echelle Spectrograph (VLT/UVES) spectra and Transiting Exoplanet Survey Satellite (TESS) photometric observations of the pulsating EL CVn binary WASP 1021-28, containing a He-core white dwarf precursor (pre-He WD). Double-lined radial velocities were measured with the atmospheric parameters of $T_{\rm eff,A}$ = 7411$\pm 40$ K, [M/H] = 0.34 $\pm$ 0.05 dex, and $v_{\rm A}$$\sin i$ = 86.6 $\pm$ 4.0 km s$^{-1}$ for the more massive primary. Combining these measurements and TESS data from four sectors allowed the direct calculation of accurate values for the absolute parameters of each component and the distance to the system. The third-light source of $l_3$ = 0.029 may be the outer tertiary object previously discovered by Spectro-Polarimetric High contrast Exoplanet REsearch/IR Differential Imaging Spectrometer (SPHERE/IRDIS) observations. Wide Angle Search for Planets (WASP) 1021-28 A is located near the blue edge of the $\gamma$ Dor instability strip, and the less massive companion is concurrent with the He-core WD model for metallicity Z = 0.02 and mass M = 0.191 ${\rm M}_\odot$. The Z-value and the Galactic kinematics demonstrate that the programme target belongs to the thin-disc population. We iteratively pre-whitened the entire TESS residuals and extracted four and nine significant signals in two ranges of 1.12–2.25 d$^{-1}$ and 111.25–139.24 d$^{-1}$, respectively. A signal of $f_2$ = 1.31865 d$^{-1}$ in the low-frequency region can be attributed to the $\gamma$ Dor pulsation of WASP 1021-28 A, and the high frequencies may be extremely low-mass pre-He WD oscillations. The results presented here provide valuable information on the evolution of short-period EL CVn stars proposed as inner binaries of hierarchical triple systems and the multiperiodic pulsations.