2022ApJ...936...33Z

We report a single-lined white dwarf-main-sequence binary system, LAMOST J172900.17+652952.8, which is discovered by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST)'s medium-resolution time-domain surveys. The radial-velocity semi-amplitude and orbital period of the optical visible star are measured by using follow-up observations with the Palomar 200 inch telescope and light curves from the Transiting Exoplanet Survey Satellite (TESS). Thus the mass function of the invisible candidate white dwarf is derived, f(M₂) = 0.120 ± 0.003 M_⊙. The mass of the visible star is measured based on a spectral energy distribution fitting, M₁ = ${0.81}_{-0.06}^{+0.07}\,{M}_{\odot }$. Hence, the mass of its invisible companion is M₂ ≳ 0.63 M_⊙. The companion ought to be a compact object rather than a main-sequence star owing to the mass ratio q = M₂/M₁ ≳ 0.78 and the single-lined spectra. The compact object is likely to be a white dwarf if the inclination angle is not small, i ≳ 40°. By using the Galaxy Evolution Explorer (GALEX) near-UV flux, the effective temperature of the white dwarf candidate is constrained as ${T}_{\mathrm{eff}}^{\mathrm{WD}}$ ≲ 12,000-13,500 K. It is difficult to detect white dwarfs which are outshone by their bright companions via single-epoch optical spectroscopic surveys. Therefore, optical time-domain surveys can play an important role in unveiling invisible white dwarfs and other compact objects in binaries.