  Over the course of three hours on 27 December 2008 we obtained optical
(R-band) observations of the blazar S5 0716+714 at a very fast cadence of 10 s.
Using several different techniques we find fluctuations with an approximately
15-minute quasi-period to be present in the first portion of that data at a > 3
sigma confidence level. This is the fastest QPO that has been claimed to be
observed in any blazar at any wavelength. While this data is insufficient to
strongly constrain models for such fluctuations, the presence of such a short
timescale when the source is not in a very low state seems to favor the action
of turbulence behind a shock in the blazar's relativistic jet.
