Optical quasi-periodic oscillation of the BL Lacertae object S5 0716+714
  during the faint state
  In order to search for the evidence of quasi-periodic oscillation (QPO) in
blazar, multicolor optical observations of the BL Lacertae object S5 0716+714
were performed from 2005 to 2012. For $I$ band on March 19 2010 with low
variability amplitude and low flux level, the same quasi-periodic oscillation
$\simeq50$ minutes with 99% significance levels is confirmed by ZDCF method,
Lomb-Scargle method, REDFIT and fitting sinusoidal curves. The observed QPO is
likely to be explained by accretion disk variability. If the observed QPO
indicates an innermost stable orbital period from the accretion disk, the QPO
$\simeq50$ min corresponds to a black hole mass of $5.03\times10^6 M_\odot$ for
a non-rotating Schwarzschild black hole and $3.2\times10^7 M_\odot$ for a
maximally rotating Kerr black hole.
