  We present the results of an observational photo-polarimetry campaign of the
blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at
very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov
telescopes.
  Our high-temporal resolution data show significant variations in the linear
polarization percentage and position angle at inter-night time-scales, while at
shorter (intra-night) time-scales both parameters varied less significantly, if
at all. Changes in the polarization angle seem to be common in gamma-ray
emitting blazars. Simultaneous differential photometry (through the B and R
bands) shows no significant variability in the total optical flux. We provide B
and R magnitudes, along with a finding chart, for a set of field stars suitable
for differential photometry.
