Polarization properties of OH masers in AGB and post-AGB stars
  Context: Ground-state OH maser emission from late-type stars is usually
polarized and remains a powerful probe of the magnetic field structure in the
outer regions of circumstellar envelopes if observed with high angular and
spectral resolutions. Observations in all four Stokes parameters are quite
sparse and this is the most thorough, systematic study published to date.
  Aims: We aim to determine polarization properties of OH masers in an
extensive sample of stars that show copious mass loss and search for candidate
objects that are well-suited for high angular resolution studies.
  Methods: Full-polarization observations of the OH 1612 and 1667 MHz maser
transitions were carried out for a sample of 117 AGB and post-AGB stars.
Several targets were also observed in the 1665 MHz line.
  Results: Polarized features occur in more than 75% of the sources in the
complete sample and there is no intrinsic difference in the occurrence of
polarized emission between the three classes of objects of different infrared
characteristics. The highest fractional polarization occurs for the post-AGB+PN
and the Mira+SR classes at 1612 and 1667 MHz, respectively. Differences in the
fractional polarization between the sources at different evolutionary stages
appear to be related to depolarization caused by blending. The alignment of the
polarization angles at the extreme sides of the shell implies a regular
structure of the magnetic field of a strength of 0.3-2.3 mG.
  Conclusions: Polarized OH maser features are widespread in AGB and post-AGB
stars. The relationship between the circular and linear fractional
polarizations for a representative sample are consistent with the standard
models of polarization for the Zeeman splitting higher than the Doppler line
width, whereas the polarized features are the sigma components.
