We assess the capability of the Chinese Space Station Telescope (CSST) to detect wide binaries in the ultra-faint dwarf Segue I, and to distinguish between cusped and cored dark matter haloes using the two-point correlation function of stellar pairs. CSST can robustly detect wide binaries at the 3σ level for binary fractions ≥1% with ~2300 stars, but distinguishing profiles requires ~6000 stars and binary fraction ≥10% within ~40 kpc.

Using 116 wide binary systems as coeval, chemically homogeneous pairs, we isolate the factors governing lithium depletion. Effective temperature (T_eff) is the dominant parameter, recovering the Li dip, plateau, and a linear trend for cool stars. Projected rotational velocity (v sin i) is not an independent driver. We identify one anomalous system with a ~1.4 dex Li excess, possibly due to planetesimal accretion or binary interaction with a tertiary companion.

We calculate the tidal evaporation rate of wide binaries under gravitational perturbations from randomly distributed, spatially extended dark solitons. Using high-probability halo-like binary candidates from Gaia EDR3 with separations >0.1 pc, we derive constraints on axion-like particle solitons. Survival of these binaries rules out soliton masses above a few solar masses for sizes ~0.01–0.1 pc, probing boson masses around 10⁻¹⁷–10⁻¹⁵ eV.

Using 1308 FGK+M wide binaries observed by LAMOST, we calibrate M dwarf [Fe/H] via the data-driven SLAM model. The training labels come from the FGK companions, covering [Fe/H] from −1 to +0.5 dex and T_eff from 3100 to 4400 K. The model yields uncertainties of ~0.15 dex in [Fe/H] and ~40 K in T_eff at SNR~100. We apply it to ~63,000 M dwarfs, and compare with other calibrations, finding good consistency with Birky et al. (2020) and a systematic offset relative to APOGEE.

We analyze 4050 wide binaries containing a white dwarf (WD) and a main-sequence (MS) star. Using BASE-9 modelling, we determine ages for 3551 WDs with median precision σ_τ/τ ~20%. Ages are validated against known clusters and WD-WD binaries. This provides precise ages for low-mass MS companions, otherwise inaccessible. The catalog includes ages, ZAMS masses, extinctions, and distance moduli.

We construct a pure sample of ultrawide binaries (0.01–1 pc) from Gaia DR2, divided into disk-like, intermediate, and halo-like populations by tangential velocity. The separation distribution follows a power law p(s) ∝ s^−1.54 for s < 10⁴ au, but steepens at larger separations. The steepening is stronger for halo binaries than disk binaries, providing constraints on formation scenarios and possible MACHO disruption.

Using 42,000 wide binaries from Gaia DR2, we measure the mass ratio distribution p(q) for primary masses 0.1–2.5 M_⊙ and separations 50–50,000 au. We discover a sharp excess of equal-mass “twin” binaries (q ≳ 0.95) persisting out to 1000–10,000 au. The twin excess decreases with separation but its width (q ≳ 0.95) is constant over 0.01–10,000 au. We argue that wide twins likely formed at closer separations and were subsequently widened by dynamical interactions.