  Spatio-temporally chaotic dynamics of transitional plane Couette flow may
give rise to regular turbulent-laminar stripe patterns with a large-scale
pattern wavelength and an oblique orientation relative to the laminar flow
direction. A recent dynamical systems analysis of the oblique stripe pattern
demonstrated that the Navier-Stokes equations have unstable equilibrium
solutions that capture the three-dimensional spatial structure of the oblique
stripe patterns. Here we identify unstable periodic orbits that not only show
oblique large-scale amplitude modulation but also have a characteristic
time-evolution, unlike steady equilibrium solutions. The periodic orbits
represent standing waves that slowly propagate across wavy velocity streaks in
the flow on viscous diffusion time scales. The unstable periodic orbits are
embedded in the edge of chaos in a symmetry subspace of plane Couette flow and
thereby may mediate transition to and from turbulent flows with oblique
patterns.
