Stellar Multiplicity of M Dwarfs with Short-period Giant Planets, and the Characterization of TOI-5628Ab

Binary stars are ubiquitous, yet it remains unclear how wide-orbit stellar companions influence the formation of hot Jupiters, particularly around M dwarfs. Here, we first report the discovery of TOI-5628Ab, a giant planet transiting a mid-type M dwarf (M_∗=0.36±0.02 M_⊙) every 4.34 days, accompanied by an associated white dwarf TOI-5628B (M_ WD=0.59±0.16 M_⊙) at a projected distance of about 2,500 AU. Using TESS, ground-based photometry and SPIRou RVs, we constrain the planet radius to 0.74±0.04 R_J and mass to 0.09±0.04 M_J, with a 3σ upper limit of 0.22 M_J. Building on this system, we further conduct a homogeneous systematic search for co-moving stellar companions with projected semi-major axis between 100 and 10,000 AU around all M dwarfs with confirmed giant planets with periods smaller than 10 days and radii larger than 0.7 R_J, as well as a group of field M stars with stellar properties similar to the planet sample, based on the stellar kinematics from Gaia DR3. We measure a stellar multiplicity of 34.2±9.5% for M dwarfs hosting short-period giant planets, which is substantially higher than the fraction of 5.3±3.7% for the field M stars by approximately a factor of 6. Our results suggest that wide-orbit stellar companions tend to promote the formation of short-period giant planets around M stars with masses 0.21 ≤ M_∗≤ 0.64 M_⊙, and high-eccentricity migration may play an important role in producing such systems.