2023AJ....165..119Y

The extremely-low-mass white dwarfs (ELM WDs) and pre-ELM WDs are helium-core white dwarfs with mass <∼ 0.3M_⊙. Evolution simulations show that a lower mass limit for ELM WDs exists at ≈0.14M_⊙ and no ELM WD is proposed by observation to be less massive than that. Here we report the discovery of a binary system, LAMOST J224040.77-020732.8 (J2240 in short), which consists of a very low-mass hot star and a compact companion. Multiepoch spectroscopy shows an orbital period P_orb = 0.219658 ± 0.000002 days and a radial-velocity semiamplitude K1 = 318.5 ± 3.3 km s^–1, which gives the mass function of 0.74M_⊙, indicating the companion is a compact star. The F-type low-resolution spectra illustrate no emission features, and the temperature (∼7400 K) is consistent with that from spectral energy distribution fitting and multicolor light-curve solution. The optical light curves, in ZTF g, r, and i bands and the Catalina V band, show ellipsoidal variability with amplitudes of ∼30%, suggesting that the visible component is heavily tidally distorted. Combining the distance from Gaia survey, the ZTF light curves are modeled with Wilson-Devinney code and the result shows that the mass of the visible component is $M1={0.085}_{-0.024}^{+0.036}{M}_{\odot }$, and the mass of the invisible component is $M2={0.98}_{-0.09}^{+0.16}{M}_{\odot }$. The radius of the visible component is $R1={0.29}_{-0.03}^{+0.04}{R}_{\odot }$. The inclination angle is approximately between 60° and 90°. The observations indicate the system is most likely a pre-ELM WD + WD/NS binary, and the mass of pre-ELM is possibly lower than the 0.14M_⊙ theoretical limit.