Statistics of the fractional polarisation of compact radio sources in
  Planck maps
  In this work we apply the stacking technique to estimate the average
fractional polarisation from 30 to 353 GHz of a primary sample of 1560 compact
sources - essentially all radio sources - detected in the 30 GHz Planck all-sky
map and listed in the second version of the Planck Catalogue of Compact Sources
(PCCS2). We divide our primary sample in two subsamples according to whether
the sources lay (679 sources) or not (881 sources) inside the sky region
defined by the Planck Galactic mask (fsky ~ 60 per cent) and the area around
the Magellanic Clouds. We find that the average fractional polarisation of
compact sources is approximately constant (with frequency) in both samples
(with a weighted mean over all the channels of 3.08 per cent outside and 3.54
per cent inside the Planck mask). In the sky region outside the adopted mask,
we also estimate the {\mu} and {\sigma} parameters for the log-normal
distribution of the fractional polarisation, finding a weighted mean value over
all the Planck frequency range of 1.0 for {\sigma} and 0.7 for {\mu} (that
would imply a weighted mean value for the median fractional polarisation of 1.9
per cent).
