  Aims. Core-collapse supernovae may show significant polarization that implies
non-spherically symmetric explosions. We observed the type II-plateau SN 2005af
using optical polarimetry in order to verify whether any asphericity is present
in the supernova temporal evolution. Methods. We used the IAGPOL imaging
polarimeter to obtain optical linear polarization measurements in R (five
epochs) and V (one epoch) broadbands. Interstellar polarization was estimated
from the field stars in the CCD frames. The optical polarimetric monitoring
began around one month after the explosion and lasted ~30 days, between the
plateau and the early nebular phase. Results. The weighted mean observed
polarization in R band was [1.89 +/- 0.03]% at position angle (PA) 54 deg.
After foreground subtraction, the level of the average intrinsic polarization
for SN 2005af was ~0.5% with a slight enhancement during the plateau phase and
a decline at early nebular phase. A rotation in PA on a time scale of days was
also observed. The polarimetric evolution of SN 2005af in the observed epochs
is consistent with an overall asphericity of ~20% and an inclination of ~30
deg. Evidence for a more complex, evolving asphericity, possibly involving
clumps in the SN 2005af envelope, is found.
