Letter to the Editor

The massive hot subdwarf binary LAMOST J065816.72+094343.1

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28 Karl-Liebknecht-Str. 24/25 D-14476 Potsdam-Golm, Germany
² Department of Physics and Astronomy, University of North Carolina 120 E. Cameron Avenue Chapel Hill NC 27599, USA
³ Yunnan Observatories, Chinese Academy of Sciences Kunming 650011, PR China
⁴ International Centre of Supernovae (ICESUN), Yunnan Key Laboratory Kunming 650216, China
⁵ University of Chinese Academy of Sciences Beijing 100049, China
⁶ Thüringer Landessternwarte Tautenburg Sternwarte 5 D-07778 Tautenburg, Germany
⁷ Hamburger Sternwarte, University of Hamburg Gojenbergsweg 112 21029 Hamburg, Germany
⁸ Department of Physics and Astronomy, Texas Tech University Lubbock TX 79409-1051, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 November 2025
Accepted: 18 December 2025

Abstract

Context. Massive short-period binaries involving hot subdwarf stars (sdO/Bs) are rare but very relevant to constraining pathways for binary star evolution. Moreover, some of the most promising candidate progenitor systems leading to Type Ia supernovae (SNe Ia) involve sdO/Bs. LAMOST J065816.72+094343.1 has been identified to be such a candidate system.

Aims. To explore the nature and evolutionary future of LAMOST J065816.72+094343.1, we complemented archival spectroscopic data with additional time series spectra as well as high-resolution spectroscopy of the object. After combining these with photometric data, we determined the orbital parameters of the system and the mass of the companion.

Methods. We solved the orbit of the system by analyzing 68 low- and medium-resolution spectra using state-of-the-art mixed local thermodynamic equilibrium (LTE) and non-LTE model atmospheres. Additionally, we gathered nine high-resolution spectra to determine atmospheric parameters and the projected rotational velocity of the sdOB. The inclination angle of the system was constrained assuming tidal synchronization of the sdOB, which was verified via analysis of the ellipsoidal variations in the TESS light curve.

Results. We determine LAMOST J065816.72+094343.1 to be a binary consisting of a massive 0.82 ± 0.17 M_⊙ sdOB component with a 1.30^+0.31_−0.26 M_⊙ unseen companion. Due to the companion’s mass being very close to the Chandrasekhar mass limit and high for a white dwarf, it is unclear whether the compact companion is a white dwarf or a neutron star. We find the system to be in a close orbit, with a period of P = 0.31955193 d and an inclination angle of i = 49.6^+5.2_−4.2 deg. While the exact nature of the companion remains unknown, we determine the system to either lead to a SN Ia or an intermediate mass binary pulsar, potentially after a phase as an intermediate-mass X-ray binary.