SIMBAD references

We argue that inner rings in barred spiral galaxies are associated with specific 2D and 3D families of periodic orbits located just beyond the end of the bar. These are families located between the inner radial ultraharmonic 4:1 resonance and corotation. They are found in the upper part of a type-2 gap of the x1 characteristic, and can account for the observed ring morphologies without any help from families of the x1-tree. Due to the evolution of the stability of all these families, the ring shapes that are favoured are mainly ovals, as well as polygons with `corners' on the minor axis, on the sides of the bar. On the other hand, pentagonal rings, or rings of the NGC 7020-type hexagon, should be less probable. The orbits that make the rings belong in their vast majority to 3D families of periodic orbits and orbits trapped around them.