2024MNRAS.533.2058C

We present the Transiting Exoplanet Survey Satellite (TESS) photometry and high-resolution spectra of the EL CVn-type systems, which consists of an A/F-type main-sequence (MS) star and a hotter low-mass helium white dwarf precursor (pre-He WD). In the spectral analysis, double-lined radial velocities (RVs) were obtained for the first time. From a simultaneous analysis of the TESS light and RV curves, the masses, radii, and temperatures of each component were determined. We applied multiple frequency analyses to the residual light curve after subtracting the binarity effects and detected independent frequencies. We found that the low-frequency region's frequencies resembled those of $\delta$ Sct-type stars, while the high-frequency region's frequencies (100-300 d$^{-1}$) likely correspond to pre-He WD components. According to our research, we found that in nine systems, the primary star with higher mass showed pulsation characteristics similar to a $\delta$-Sct-type. In only three systems, the hotter secondary components consisted of a pre-He WD with high-frequency pulsations. We examine the purity of pre-He WD instability strip by studying several EL CVn-type systems. Additionally, we propose new boundaries for the empirical instability strips of pre-He WDs.