2025AJ....170..253L

Strottner-Drechsler-Sainty Object 1 (SDSO1) is a faint [O III] emission nebula spanning 1$\mathop{.}\limits^{^\circ }5$, discovered southeast of M31. In previous surveys, no counterpart has been observed across radio, infrared, UV, and X-ray wavelengths, thus complicating distance determination. Utilizing the M31 H I survey with the Five-hundred-meter Aperture Spherical radio Telescope, we identified an atomic gas structure associated with SDSO1. This potential neutral gas counterpart includes three strips spatially aligned with the [O III] emission arcs. The H I structure has a velocity between -79 and -45 km s^–1 with respect to the local standard of rest. It angularly extends to $2\mathop{.}\limits^{^\circ }9$, surpassing SDSO1 in size. Several structures around the H I counterpart share similar velocities, hinting at a link between the H I counterpart and the Galactic disk's large-scale structure. The combined analysis of the three-dimensional dust map and the reddening-distance relations for these H I strips and their nearby H I structures suggests a distance of 2.0 ± 0.2 kpc. The mass of the H I counterpart is calculated to be 614M_⊙. All of these results collectively demonstrate that SDSO1 is most likely a structure in the Milky Way rather than one associated with M31.