  We investigate the dynamical response of stellar orbits in a rotating barred
galaxy potential to the perturbation by a nuclear gaseous ring. The change in
3D periodic orbit families is examined as the gas accumulates near the inner
Lindblad resonance. It is found that the x2/x3 loop extends to higher Jacobi
energy and a vertical instability strip forms in each family. These strips are
connected by a symmetric/anti-symmetric pair of 2:2:1 3D orbital families. A
significant distortion of the x1 orbits is observed in the vicinity of the
ring, which leads to the intersection between orbits over a large range of the
Jacobi integral. We also find that a moderately elliptical ring oblique to the
stellar bar produces significant phase shifts in the x1 orbital response.
