The pre-He white dwarfs in eclipsing binaries – IV. WASP 1814+48 with multiperiodic pulsations

Abstract

ABSTRACT 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 Teff, 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).