H i Effelsberg spectra (left) and SDSS fields (right) of the detected (including marginal) XMP galaxies. The H i spectra of J0014-0044 is dominated by the H i gas of the nearby spiral galaxy UGC 139 (Sect. 2.2.2). In the spectra, the blue vertical solid lines mark the radial velocity from SIMBAD (heliocentric; converted from the optical to the radio convention), the blue vertical dashed lines mark the regions for the baseline computation and flux density integration, and the red lines represent a smoothed version of the spectrum, using a Gaussian kernel of width σ_sm = 21.22  km   s^-1. The smoothing kernel was chosen such that the resulting spectral resolution becomes σ_res = 21.25  km   s^-1, which is optimally suited to detect a (Gaussian) feature of line width (FWHM). Note that the integrated flux densities and limits were derived from the full-resolution spectra/rms.