2025A&A...693A.322Y

We present an analysis of LAMOST J171013.211+532646.04 (hereafter J1710), a binary system comprising a hot subdwarf B star (sdB) and a white dwarf (WD) companion. Multi-epoch spectroscopy revealed an orbital period of 109.20279 minutes, consistent with TESS and ZTF photometric data. This means that this is the sixth detached system known to harbor a WD companion with a period shorter than two hours. J1710 is remarkably close to Earth. It is situated at a distance of only $ 350.68^{+4.20}_{-4.21} \, \mathrm{pc} $, with a Gaia G-band magnitude of 12.59. This renders it conducive for continuous observations. The spectral temperature is around 25 164 K, in agreement with fitting results ($ 25301^{+839}_{-743} \, \mathrm{K} $) based on a spectral energy distribution. The TESS light curve displays ellipsoidal variation and Doppler beaming without eclipsing features. Through fitting the TESS light curve using the Wilson-Devinney code, we determined the masses for the sdB ($ M_1 = 0.44^{+0.06}_{-0.07} \, M_{\odot} $) and the compact object ($ M_2 = 0.54^{+0.10}_{-0.07} \, M_{\odot} $); the compact object likely is a WD. Furthermore, MESA models suggest that the sdB, with a helium core mass of 0.431 M_⊙ and a hydrogen envelope mass of 1.3 × 10^–3 M_⊙, is in the early helium main-sequence phase. The MESA binary evolution shows that the J1710 system is expected to evolve into a double WD system. This means that it is an important source of low-frequency gravitational waves.