Ultra-diffuse galaxies, an extreme type of dwarf galaxy, have been the focus of extensive observational and theoretical studies over the past decade. With stellar masses comparable to dwarf galaxies (between 10 7 and 10 9 solar masses) but much larger in size (as defined by their effective radius), they exhibit an extremely low surface brightness. These galaxies display highly diverse properties: some have large dark matter halos, others lack them, and their number of globular clusters varies widely. Studies of their kinematics and stellar populations have shown that these extreme galaxies

WISEA J181006.18-101000.5 (WISE1810) is the nearest metal-poor ultracool dwarf to the Sun. It has a low effective temperature and has been classified as an extreme early-T subdwarf. However, methane--the characteristic molecule of the spectral class T--was not detected in the previous low-resolution spectrum. Constraining the metallicity--the abundance of elements heavier than helium-- of these cold objects has been a challenge. Using the 10.4 m Gran Telescopio Canarias, the largest optical-infrared telescope in the world, we collected a high-quality near-infrared intermediate-resolution