  We present the results of a VLT polarimetric monitoring campaign of the GRB
020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days
after the gamma-ray event. The mean values of the degree of linear polarization
(P) and its position angle (Theta) resulting from our campaign are
<P>=1.18+/-0.10% and Theta = 148.7+/-2.3 deg after correcting for Galactic
interstellar polarization. Our VLT data set is most naturally described by a
constant degree of linear polarization and position angle, nonetheless a slow
Theta evolution cannot be entirely ruled out by our data. The VLT monitoring
campaign did not reveal either a significant Theta rotation respect to the Keck
spectropolarimetric observations performed ~0.25 days after the GRB (Barth et
al. 2003). However, <P> is a factor of two lower than the polarization degree
measured from Keck. Additionally, the VLT polarization data allowed us to
construct an accurate V-band light curve. The V-band photometric data revealed
a smooth light curve with a break located between the last Keck and our first
VLT polarimetric measurement, 0.33 < t_(break, V) < 0.88 days after the GRB.
The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag,
0.010 mag and 0.016 mag for our three observing nights, respectively. We
speculate that the stability of Theta might be related to the smoothness of the
light curve.
