Low-resolution optical spectra for our candidates (black) with identifier (ID) and telescope name. Overplotted in red, green, and blue are the spectral templates of SDSS dwarfs/subdwarfs used to assign the final spectral types (see Sect. 5.2 and legend on the plot). First and second columns: 26 candidates confirmed as extreme subdwarfs. Third column, lower part: six candidates confirmed as ultrasubdwarfs. Third column, upper part: five candidates classified as dM and two candidates with uncertain class, between dM and sdM. Candidates with IDs 5, 9, 11, 15, and 7 appear twice because they were included in our spectroscopic follow-up and have a spectrum in the SDSS spectroscopic database.