Marginal gravitational lensing

Gravitational lenses with four types of marginality are investigated. The first type concerns a definite lens able to split light rays and a range of source positions, yielding a pair of amplified images close to merging. The second is a range of lenses barely able to split images. The third and fourth types of marginality concern two-component lenses in which one component plays the role of a background (e.g., a cluster or similar gravitational field) that affects the images split by the second component (e.g., a galactic or stellar gravitational field). The order-of-magnitude estimates of characteristic image separations and amplifications for the pair of brightest images produced by the four kinds of marginal lenses are presented. 57 references