OH maser emission from the circumstellar envelope of the semiregular variab
le star R Crt has been imaged in all Stokes parameters using the MERLIN arr
ay. The 1665- and 1667-MHz emission has a compact structure with an angular
extent less than 0.2 arcsec. The combined 1665- and 1667-MHz data can be f
itted with a thin-shell model of angular radius 0.14 +/- 0.04 arcsec. There
is a systematic displacement between the blueshifted emission, which tends
to lie to the south, and the redshifted emission, which lies mainly to the
north. The polarization properties of the blue- and redshifted emission al
so differ markedly. Linear polarization is observed in the blueshifted 1667
-MHz emission only, while there are spatial separations between the maxima
of left and right circular polarization for the blueshifted emission. It is
suggested that magnetic effects may be responsible both for the polarizati
on asymmetries and for the front-to-back asymmetries in the OH envelope.