Abstract The determination of unseen companion masses ( M 1 ) is essential for identifying compact objects in binary systems, yet obtaining reliable orbital inclinations remains one of the most difficult challenges. In this study, we focus on 10 targets selected from a sample of 89 compact object candidates characterized by large mass functions, rapid rotation, and high-quality Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) spectra. We measure their projected rotational velocities ( v sin i ) from the LAMOST medium-resolution spectra and, combined with stellar radii, derive orbital inclinations and the corresponding companion masses. Our results show that five sources exhibit mass ratios M 1 / M 2 > 2/3, with no detectable spectral signatures of the unseen companions, providing strong evidence for their compact nature. Two particularly notable cases, J0341 and J0359, host companions with inferred masses of 1.3 9 − 0.10 + 0.09 M ⊙ and 1.3 4 − 0.09 + 0.08 M ⊙ , respectively. These masses suggest that the invisible objects are either neutron stars or massive white dwarfs with masses close to the Chandrasekhar limit. If they are white dwarfs, these two targets are highly likely to be Type Ia supernova progenitors. This study highlights the potential of v sin i measurements as a systematic approach to unveiling compact objects in binaries.
Wang et al. (2026) studied this question.