LAMOST J171013+532646: a detached short-period non-eclipsing hot subdwarf + white dwarf binary

Abstract: 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 reveals an orbital period of 109.20279 minutes, consistent with TESS and ZTF photometric data, marking it as the sixth detached system known to harbor a WD companion with a period less than two hours. J1710 is remarkably close to Earth, situated at a distance of only (350.68^{+4.20}_{-4.21} \, \mathrm{pc}), with a GAIA G-band magnitude of 12.59, rendering it conducive for continuous observations. The spectral temperature is around 25164 K, in agreement with SED fitting results ((25301^{+839}_{-743} \, \mathrm{K})). 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})), with the compact object likely being a WD. Furthermore, MESA models suggest that the sdB, with a helium core mass of 0.431 (M_{\odot}) and a hydrogen envelope mass of (1.3 \times 10^{-3}\, M_{\odot}), is in the early helium main-sequence phase. The MESA binary evolution shows that the J1710 system is expected to evolve into a double white dwarf system, making it an important source of low-frequency gravitational waves.