The optical microvariability and spectral changes of the BL Lacertae
  object S5 0716+714
  We monitored the BL Lac object S5 0716+714 in the optical band during October
2008, December 2008 and February 2009 with a best temporal resolution of about
5 minutes in the BVRI bands. Four fast flares were observed with amplitudes
ranging from 0.3 to 0.75 mag. The source remained active during the whole
monitoring campaign, showing microvariability in all days except for one. The
overall variability amplitudes are delta B ~ 0.89 mag, delta V ~ 0.80 mag,
delta R ~ 0.73 mag and delta I ~0.51 mag. Typical timescales of
microvariability range from 2 to 8 hours. The overall V - R color index ranges
from 0.37 to 0.59. Strong bluer- when-brighter chromatism was found on
internight timescales. However, different spectral behavior was found on
intranight timescales. A possible time lag of ~ 11 mins between B and I bands
was found on one night. The shock-in-jet model and geometric effects can be
applied to explain the source's intranight behavior.
