Images form at extrema of the arrival time surface (i.e. Fermat's principle). At saddle points, images have negative and at minima/maxima, they have positive parity. In quadruply-imaged system one observes two saddle point images, and two minima. The fifth image is a maximum, too faint to be observed.

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For distance calculations throughout the paper we assume an Einstein-de-Sitter Universe and the Hubble constant $H_{0}=65\:{\rm km~ s^{-1}~ Mpc^{-1}}$ .

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...

For distance calculations throughout the paper we assume an Einstein-de-Sitter Universe and the Hubble constant $H_{0}=65\:{\rm km~ s^{-1}~ Mpc^{-1}}$ .

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