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It was noticed by one of us that the companion to the ring galaxy II Hz 4 seems embedded in a second and much fainter ring; this discovery supports a simple dynamical mechanism for ring galaxies that had recently occurred to the other. The present paper displays the new photographs and various spectroscopic data. It also illustrates the proposed mechanism with some computer experiments. In essence, after a companion galaxy has fallen right through a given disk galaxy more or less along the axis, a transient but remarkably severe and ringlike density wave develops as that target disk rebounds from the shortlived extra pull toward its center. Subject headings: galaxies: internal motions - galaxies: structure
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Lynds et al. (1976) studied this question.
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