2022MNRAS.515.4702L

For the EL CVn candidate 1SWASPJ181417.43+481117.0 (WASP 1814+48), we secured the first spectroscopic observations between 2015 April and 2021 March. Using the echelle spectra, the radial velocities (RVs) of the primary star were measured with its atmospheric parameters of T_eff, 1_ = 7770 ± 130 K and $v$₁sin i = 47 ± 6 km s^–1. We fitted our single-lined RVs and the TESS light curve simultaneously. From the binary modelling, we determined the following fundamental parameters for each component: M₁ = 1.659 ± 0.048 M_⊙, R₁ = 1.945 ± 0.027 R_⊙, and L₁ = 12.35 ± 0.90 L_⊙ for WASP 1814+48 A, and M₂ = 0.172 ± 0.005 M_⊙, R₂ = 0.194 ± 0.005 R_⊙, and L₂ = 0.69 ± 0.07 L_⊙ for WASP 1814+48 B. The surface gravity of log g₂ = 5.098 ± 0.026 obtained from M₂ and R₂ is concurrent with 5.097 ± 0.025 computed directly from the observable quantities. WASP 1814+48 B is well matched with the 0.176-M_⊙ white dwarf (WD) evolutionary model for Z = 0.01. The metallicity and our Galactic kinematics indicate that the program target is a thin-disc star. The whole light residuals after the removal of the binary trend were analysed and found to oscillate at a total of 52 frequencies. Among these, most of the low frequencies below 24 d^–1 are aliases and orbital harmonics. The five significant frequencies between 32 and 36 d^–1 are the pulsation modes of WASP 1814+48 A located in the δ Sct domain on the zero-age main sequence, and the high frequencies of 128-288 d^–1 arise from WASP 1814+48 B in the pre-He WD instability strip. Our results reveal that WASP 1814+48 is the fifth EL CVn star that is composed of a δ Sct-type primary and a pre-ELMV (extremely low-mass pre-He WD variable).