  We present the most complete atlas of nuclear rings to date. We include 113
rings found in 107 galaxies, six of which are elliptical galaxies, five are
highly inclined disc galaxies, 18 are unbarred disc galaxies, and 78 are barred
disc galaxies. Star-forming nuclear rings occur in 20% of disc galaxies with
types between T=-3 and T=7. We aim to explore possible relationships between
the size and morphology of the rings and various galactic parameters. We
produce colour index and structure maps, as well as Halpha and Paalpha
continuum-subtracted images from HST archival data. We derive ellipticity
profiles from H-band 2MASS images in order to detect bars and find their metric
parameters. We measure the non-axisymmetric torque parameter, Qg, and search
for correlations between bar, ring metric parameters, and Qg.
  Our atlas of nuclear rings includes star-forming and dust rings. Nuclear
rings span a range from a few tens of parsecs to a few kiloparsecs in radius.
Star-forming nuclear rings can be found in a wide range of morphological types,
from S0 to Sd, with a peak in the distribution between Sab and Sb, and without
strong preference for barred galaxies. Dust nuclear rings are found in
elliptical and S0 galaxies. For barred galaxies, the maximum radius that a
nuclear ring can reach is a quarter of the bar radius. We found a nearly random
distribution of PA offsets between nuclear rings and bars. There is some
evidence that nuclear ring ellipticity is limited by bar ellipticity. We
confirm that the maximum relative size of a star-forming nuclear ring is
inversely proportional to the non-axisymmetric torque parameter, Qg, and that
the origin of nuclear rings, even the ones in non-barred hosts, are closely
linked to the existence of dynamical resonances.
