  We first briefly review how we investigate the modes of oscillation trapped
within the inner region of accretion disks by the strong-field gravitational
properties of a black hole (or a compact, weakly-magnetized neutron star). Then
we focus on the `corrugation'(c)-modes, nearly incompressible perturbations of
the inner disk. The fundamental c-modes have eigenfrequencies (ordered by
radial mode number) which correspond to the Lense-Thirring frequency, evaluated
at the outer trapping radius of the mode, in the slow rotation limit. This
trapping radius is a decreasing function of the black hole angular momentum, so
a significant portion of the disk is modulated only for slowly rotating black
holes. The eigenfrequencies are thus strongly increasing functions of black
hole angular momentum. The dependence of the eigenfrequencies on the speed of
sound within (or the luminosity) within the disk is very weak, except for
slowly rotating black holes.
