Evidence for quasi-periodic oscillations in the optical polarization of   the blazar PKS 2155-304
  Evidence for the presence of quasi-periodic oscillations (QPOs) in the optical polarization of the blazar PKS 2155-304, during a period of enhanced gamma-ray brightness, is presented. The periodogram of the polarized flux revealed the existence of a prominent peak at $T\sim 13$ min, detected at >99.7% significance, and $T\sim 30$ min, which was nominally significant at >99%. This is the first evidence of QPOs in the polarization of an active galactic nucleus, potentially opening up a new avenue of studying this phenomenon. 
N. W. Pekeur, A. R. Taylor, S. B. Potter and R. C. Kraan-Korteweg
astro-ph.HE