Two additional members of a newly identified group of pulsating pre-He white dwarfs

ABSTRACT The VLT/UVES (Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph) spectroscopic and Transiting Exoplanet Survey Satellite (TESS) photometric observations provide a unique opportunity to directly study the physical properties of binary stars. Through careful examination of these observations, the essential characteristics of the eclipsing components were revealed. The primary components are located within the overlapping main-sequence region of the $\delta$ Sct and $\gamma$ Dor variables, while the secondary components closely match the low-mass white dwarf models. Multifrequency analyses were performed on the residual TESS data after removing the binarity effects. The frequencies in the range of 230–460 $\mu {\rm Hz}$ are attributed to $\delta$ Sct pulsations that come from the primary stars of the systems. On the other hand, the frequencies of 1160–2890 $\mu {\rm Hz}$ are considered to be oscillations of pre-He WDs. The investigated systems fall outside (hotter side) of the theoretical instability strip while positioned in the empirical instability strip. Increasing number of pre-ELMVs (Extremely Low-Mass Variables) discovered to have higher $T \rm _{eff}$s than theoretical models predicted and this could be caused by the different He abundances in the driving zone originated from different binary evolution scenarios.