Two unseen massive white dwarf candidates in close binaries

Abstract We report the discovery of two binary systems, each consisting of a slightly bloated G-type main-sequence star and an unseen companion, identified through photometric data from TESS and radial velocity variation from Gaia. High-resolution spectroscopy confirms orbital periods of 1.37 and $2.67$ d with circular orbits. The visible components have masses of ${\sim}0.9$–$1.0\, M_{\odot}$, while the minimum masses of the unseen companions are $1.078^{+0.058}_{-0.060}\, M_{\odot}$ and $0.943^{+0.048}_{-0.049}\, M_{\odot}$, respectively. Assuming tidal synchronization, we estimate the companion masses to be $1.13^{+0.11}_{-0.08}\, M_{\odot}$ and $1.05^{+0.15}_{-0.10}\, M_{\odot}$. The absence of detectable spectral features from the companions rules out main-sequence stars of these masses, suggesting that the unseen companions are likely O/Ne or C/O massive white dwarfs. The short orbital periods imply that these systems are post-common envelope binaries. Their subsequent evolution is uncertain, with possible outcomes including cataclysmic variables, Type Ia supernovae, or accretion-induced collapse, depending on the nature of future mass transfer.