and east is on the left, the synthesized beam is indicated at the lower left corner.[]{data-label="dust"}](StokesIb-eps-converted-to.pdf){height="7cm"}

![ALMA polarization map (at 345 GHz) showing the electric distribution of vectors. The contours are drawn in steps of \[0.01,0.05,0.1,0.3,0.5,0.7\]$\times$73.6 mJy beam$^{-1}$. The letters A,B and C indicate the three main polarized areas. The peak polarized emission, with a value of 1.13$\pm$0.13 mJy beam$^{-1}$, is found in the region B. The central star location is also indicated (CS on the map). North is on top and east is on the left, the synthesized beam is indicated on the left corner.[]{data-label="Evectors"}](ALMA_pol2-eps-converted-to.pdf){height="7.9cm"}

![image](Percentpol2-eps-converted-to.pdf){height="8.5cm"}

Observations and data reduction {#obs}
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The non-standard polarimetric observations were performed with 40$\times$12m ALMA antennae on 2016-12-28. The observations, under the code 2016.1.00196.S (P.I. Sahai), were done using frequency Band 7 with spectral windows centred at 336.495 GHz, 338.432 GHz, 348.495 GHz and 350.495 GHz, each window having 1.875 GHz bandwidth and 64 channels. The C40-3 configuration was used with a maximum baseline of 460m and the maximum recoverable scale (MRS) was 3.9. Two datasets were taken, with a total time of 58.22 min and 91.55 min respectively (including calibrations such as bandpass, flux, phase and polarization) and OH231.8+4.2 was observed for 