Simultaneous B'V'R' Monitoring of BL Lacertae Object S5~0716+714 and   Detection of Inter-Band Time Delay
  We present the results of our optical monitoring of the BL Lac object S5 0716+714 on seven nights in 2006 December. The monitoring was carried out simultaneously at three optical wavelengths with a novel photometric system. The object did not show large-amplitude internight variations during this period. Intranight variations were observed on four nights and probably on one more. Strong bluer-when-brighter chromatism was detected on both intranight and internight timescales. The intranight variation amplitude decreases in the wavelength sequence of B', R', and V'. Cross correlation analyses revealed that the variability at the $B'$ and $V'$ bands lead that at the $R'$ band by about 30 minutes on one night. 
Jianghua Wu, Markus Boettcher, Xu Zhou, Xiangtao He, Jun Ma, and   Zhaoji Jiang
astro-ph.HE