'Missing image' in gravitational lens systems?

Gravitational lens models involving extended mass distributions generally predict an odd number of images 1 , with one of the images close to the centre of the principal lens galaxy. In all the lens systems observed up to now, only an even number of images have been unambiguously detected. A number of suggestions 2–4 have been advanced to explain the absence of the ‘odd’ image, but none of these is sufficiently compelling. We suggest here that the presence of a compact nucleus with mass in the range 5 × 10 9 to 5 × 10 10 solar masses ( M ⊙ ), contained within a region of size ≲50 pc at the centre of a lens galaxy, would dim the odd image significantly without affecting the rest of the image configuration.