Rapid polarization variability in the BL Lacertae object S5 0716 + 714

We present optical polarimetry of the violently variable BL Lacertae object S5 0716 + 714, obtained over a three year period using the 6 m telescope at the Special Astrophysical Observatory in Russia. The degree of polarization is high and variable throughout the observations. With a minimum time resolution of 1 minute, significant variations on a timescale of 10-15 minutes are observed. The variations are complex and not fully sampled by this data, but they are characterized by large amplitude changes on a timescale of about 1 day superposed on smooth variations with a timescale of about 1 week. Possible periods of 12.5, 2.5, and 0.14 days have been discovered. S5 0716 + 714 is a highly compact radio source with nonthermal emission observed across the electromagnetic spectrum out to gamma rays. The optical polarization variations are consistent with relativistically beamed synchrotron emission viewed at a very small angle to the line of sight.