The number of studies of ring phenomenon in galaxies has grown in the last twenty years. The ring-like structures of SB(r) and Sa(r) galaxies with resonance rings, which are said to be related to orbital resonances in a bar or oval in the galactic gravitational potential (features also found in SAB galaxies) are also seen in wide-field, deep-sky images. The visual appearance of these objects will be hereafter referred to as "normal'' ringed galaxies - NRG herein (for more on these objects, see Buta 1995, and references quoted therein). Other examples of NRG phenomena are: (a) early-type disk galaxies from SO+ to Sa, which show interesting structures such as nuclear, inner, and outer rings, (b) the normal ring category of spirals having stages Sab or later, and (c) Sa, SO/a and SO galaxies where the ring pattern is less prominent.
Extensive studies of ring sizes of NRG have been developed by de Vaucouleurs & Buta (1980a,b), Buta & de Vaucouleurs (1982, 1983), among others. Studying the metric properties of the inner ring structures of S(r) galaxies, Pedreros & Madore (1980) found that ring diameters are rather highly accurate distance indicators. On the other hand, spiral galaxies which have had their arms tightly wound into nearly circular design, thus resembling ring-shaped structures (Arp & Madore 1986 - hereafter AM; Faúndez-Abans et al. 1992), are not considered as ring galaxies in this work.
In the last two decades, surveys, classification systems and statistics of ring galaxies have been made. As an example, in the seventies, Theys & Spiegel (1976) classified northern ring galaxies in three groups: (1) RE-objects, which have crisp, elliptical rings with photographically empty interiors, with archetypes such as Arp 146, Arp 147 and VII ZW 466; (2) RN-objects, which have an elliptical ring with an interior off-center nucleus, archetype II HZ 4; and (3) RK-objects, characterized by rings with prominent condensations or knots, archetypes being I Zw 45 and Zw 28. In the eighties, Few & Madore (1986) proposed a simple division of southern ring galaxies into two broad sub-types: P-type rings, which have a crispy knotty structure and often an off-center nucleus, and that are probably collisional (they happen to have a significant number of companions lying within a distance of about two ring diameters); and O-type rings, which have a smooth structure and a centrally located nucleus, no obvious companions, and that are likely to be resonant (R)S galaxies.
The diffuse stellar component (nucleus) of ring-shaped galaxies as a classification parameter was introduced by Faúndez-Abans et al. (1992 - hereafter FAHR). On the other hand, the FAHR criterion also includes SB and Sa ringed galaxies (Hodge 1966), which are NRG. The category 6 of the "Catalogue of Southern Peculiar Galaxies and Associations'' (Arp & Madore 1977 and AM) also comprises a wide variety of ring morphologies, more than expected by the presently known classifications. The AM catalogue contains a variety of objects which display warped structures, off-center nuclei, face-on elliptical rings, disrupted rings and so on, features that could be a result of interactions such as (i) collisions ("galaxies run through each other"; see Appleton & Marston 1997 for a recent work on optical, infrared, and radio continuum properties of a sample of collisional ring galaxies), (ii) tidal interactions ("one passes close to the other"), or (iii) merging (combination into one galaxy). To distinguish these objects from the NRG, we hereafter call these objects "Peculiar Ring Galaxies" (PRG, referred to as "peculiar" following AM catalogue's designation). It is also interesting to refer to the work of Naim & Lahav (1997) on "what a peculiar galaxy is". We have purposedly included polar and saturn-like structures for completeness of the O-type rings of Few & Madore (1986), even though O-type objects are perhaps resonant.
The main objective of this paper is to introduce a complementary view of PRG, based on the morphologies of the ring and the nucleus, and when possible, the scenario of the apparently close companions. The word "nucleus" was introduced by Theys & Spiegel (1976) and Few & Madore (1986); we will be using "bulge" herein, instead.