All Figures

Figure 1: Polarization spectra for the 1612 MHz transition in IRAS 18276-1431 (OH17.7-2.0) taken on 2002 February 20. Upper panel: the total flux (I Stokes) (black/blue line) and the circularly polarized flux (V Stokes) (grey/red line); middle panel: the percentage linear polarization () (black/blue line) and the percentage circular polarization () (black/blue line); lower panel: the linearly polarized flux (  Stokes) (black/blue line) and the position angle (PA) of the polarization vector (crosses). The grey/red line in the lower panel shows the flux-weighted PA of polarization vector observed with MERLIN on 1999 May 19 (Bains et al. 2003a).

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In the text

	Figure 2: Histograms of the integrated flux density for sources with polarized (solid line) and non-polarized (dashed line) OH emission.
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In the text

	Figure 3: Polarized flux densities (|V| and p) plotted against the total I flux densities. The filled symbols show sources with linearly (grey/red) and circularly (black/blue) polarized features. The open symbols show sources with only an upper limit for polarizedemission.
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In the text

	Figure 4: V Stokes 1612 MHz OH spectra of PPN candidates with possible Zeeman splitting marked by dashed ovals.
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In the text

Figure 5: Percentage of linear polarization versus the normalized total flux at 1612 MHz for sources with p flux higher than 5 seen in more than 35 spectral channels. Each point represents data for channel of width 0.07 or 0.14 km s-1, and 0.28 km s-1 for IRAS 19114+0002 (see Table 1). The line of slope of -0.57 is the best fit to the data marked by open symbols. Note the absence of decreasing trends for targets marked by small symbols.

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In the text

	Figure 6: Histogram of the ratio of the spectral maser extents at 1612 and 1667 MHz.
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In the text

	Figure 7: Position angle of the magnetic field vector versus position angle of the long axis of the nebula. Squares and circles mark the 1612 and 1667 MHz masers respectively. The diagonal dotted lines correspond to constant differences of -90, 0 and 90 between the two position angles. The inset shows the distribution of the absolute difference in PA.
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In the text

Figure A.1: Same as in Fig. 1 for the 1612 MHz transition in IRAS 07331+0021 taken on 2002, July 5.


In the text

Figure A.2: Same as in Fig. 1 for the 1667 MHz transition in IRAS 07399-1435 taken on 2002, June 16.


In the text

Figure A.3: Same as in Fig. 1 for the 1612 MHz transition in IRAS 08005-2356 taken on 2002, May 15.


In the text

Figure A.4: Same as in Fig. 1 for the 1612 MHz transition in IRAS 16342-3814 taken on 2003, April 28. There is not linearly polarized emission at the red-shifted part of the spectrum and only the blue-shifted part is shown.


In the text

Figure A.5: Same as in Fig. 1 for the 1667 MHz transition in IRAS 16342-3814 taken on 2003, April 28. There is not linearly polarized emission at the red-shifted part of the spectrum and only the blue-shifted part is shown.


In the text

Figure A.6: Same as in Fig. 1 for the 1612 MHz transition in IRAS 17150-3224 taken on 2003, March 19.


In the text

Figure A.7: Same as in Fig. 1 for the 1667 MHz transition in IRAS 17150-3224 taken on 2003, March 19.


In the text

Figure A.8: Same as in Fig. 1 for the 1612 MHz transition in IRAS 17375-2759 taken on 2003, June 27.


In the text

Figure A.9: Same as in Fig. 1 for the 1612 MHz transition in IRAS 17385-3332 taken on 2002, September 12.


In the text

Figure A.10: Same as in Fig. 1 for the 1612 MHz transition in IRAS 17393-2727 taken on 2003, February 1.


In the text

Figure A.11: Same as in Fig. 1 for the 1667 MHz transition in IRAS 17393-2727 taken on 2003, February 1.


In the text

Figure A.12: Same as in Fig. 1 for the 1612 MHz transition in IRAS 17393-3004 taken on 2003, June 30.


In the text

Figure A.13: Same as in Fig. 1 for the 1612 MHz transition in IRAS 17404-2713 taken on 2003, February 1.


In the text

Figure A.14: Same as in Fig. 1 for the 1667 MHz transition in IRAS 17404-2713 taken on 2003, February 1.


In the text

Figure A.15: Same as in Fig. 1 for the 1612 MHz transition in IRAS 17579-3121 taken on 2003, February 12.


In the text

Figure A.16: Same as in Fig. 1 for the 1612 MHz transition in IRAS 18016-2743 taken on 2002, May 24.


In the text

Figure A.17: Same as in Fig. 1 for the 1667 MHz transition in IRAS 18091-1815 taken on 2002, June 10.


In the text

Figure A.18: Same as in Fig. 1 for the 1612 MHz transition in IRAS 18135-1456 taken on 2003, February 23.


In the text

Figure A.19: Same as in Fig. 1 for the 1612 MHz transition in IRAS 18266-1239 taken on 2002, August 12.


In the text

Figure A.20: Same as in Fig. 1 for the 1667 MHz transition in IRAS 18276-1431 taken on 2002, February 20.


In the text

Figure A.21: Same as in Fig. 1 for the 1612 MHz transition in IRAS 18450-0148 taken on 2003, May 17.


In the text

Figure A.22: Same as in Fig. 1 for the 1667 MHz transition in IRAS 18450-0148 taken on 2003, May 17.


In the text

Figure A.23: Same as in Fig. 1 for the 1667 MHz transition in IRAS 18596+0315 taken on 2002, June 4.


In the text

Figure A.24: Same as in Fig. 1 for the 1612 MHz transition in IRAS 19067+0811 taken on 2003, February 23.


In the text

Figure A.25: Same as in Fig. 1 for the 1612 MHz transition in IRAS 19114+0002 taken on 2003, June 18.


In the text

Figure A.26: Same as in Fig. 1 for the 1667 MHz transition in IRAS 19114+0002 taken on 2003, June 18.


In the text

Figure A.27: Same as in Fig. 1 for the 1612 MHz transition in IRAS 19219+0947 taken on 2003, May 15.


In the text

Figure A.28: Same as in Fig. 1 for the 1612 MHz transition in IRAS 19255+2123 taken on 2002, May 25.


In the text

Figure A.29: Same as in Fig. 1 for the 1612 MHz transition in IRAS 22036+5306 taken on 2002, March 1.


In the text