Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS – IV. Discovery of two new GW Vir stars: TIC 0403800675 and TIC 1989122424

Abstract

ABSTRACT We present two new GW Vir-type pulsating white dwarf stars, TIC 0403800675 (WD J115727.68-280349.64) and TIC 1989122424 (WD J211738.38-552801.18) discovered in the Transiting Exoplanet Survey Satellite (TESS) photometric data. For both stars, the TESS light curves reveal the presence of oscillations with periods in a narrow range between 400 and 410 s, which are associated with typical gravity (g)-modes. Follow-up ground-based spectroscopy shows that both stars have similar effective temperature ($T_\mathrm{eff} = 110\, 000 \pm 10\, 000$ K) and surface gravity (log g = 7.5 ± 0.5), but different He/C composition (mass fractions): He = 0.75 and C = 0.25 for TIC 0403800675, and He = 0.50 and C = 0.50 for TIC 1989122424. By performing a fit to their spectral energy distributions, we found for both stars radii and luminosities of $R=0.019\pm 0.002\, \mathrm{R}_\odot$ and $\log (\mathrm{L/L}_\odot)=1.68^{+0.15}_{-0.24}$, respectively. By employing evolutionary tracks of PG 1159 stars, we find the masses of both stars to be $0.56\pm 0.18 \, \mathrm{M}_{\odot }$ from the log g-Teff diagram and $0.60^{+0.11}_{-0.09} \, \mathrm{M}_{\odot }$ from the Hertzsprung Russell diagram.