5. CONFUSION OF RING AND LENS TYPES IN NONBARRED GALAXIES

The classification of rings, pseudorings, and lenses into subclasses such as ``inner'', ``outer'', or ``nuclear'' is not always straightforward. Only for barred galaxies is it usually possible to unambiguously distinguish these different feature types. This is because the bar usually fills an inner ring or lens, if present, in one dimension. Occasionally, galaxies which appear to be nonbarred on a B-band image reveal a weak bar on a redder passband image, so that inner and outer features can still be distinguished. However, in some clearly nonbarred galaxies with only a single ring, it can be difficult to decide whether a ring or lens is an inner or an outer type. A confusion between inner rings and equivalent nuclear rings in nonbarred galaxies undoubtedly occurs also. Nuclear rings are easily distinguished from inner rings in SB galaxies, again because the bar usually fills the inner ring in one dimension. Without this diagnostic, however, one cannot readily distinguish an inner from a nuclear ring in an SA galaxy unless all three feature types are present. The ambiguity means that some features in SA galaxies which are really analogues of nuclear rings will be misclassified as inner rings. The inner rings in NGC 6753 (Buta and Crocker 1993) and NGC 7217 (Buta et al. 1995b) are probably of this nature. The high surface brightness ring in NGC 7742, a very axisymmetric galaxy, has been interpreted as a nuclear ring by Buta & Crocker (1993) and Wakamatsu et al. (1996).