Context : The majority of the observed planetary nebulae exhibit elliptical or bipolar structures .
Recent observations have shown that asymmetries already start during the last stages of the AGB phase .
Theoretical modeling has indicated that magnetically collimated jets may be responsible for the formation of the non-spherical planetary nebulae .
Direct measurement of the magnetic field of evolved stars is possible using polarization observations of different maser species occurring in the circumstellar envelopes around these stars .

Aims : The aim of this project is to measure the Zeeman splitting caused by the magnetic field in the OH and H _ { 2 } O maser regions occurring in the circumstellar envelope and bipolar outflow of the evolved star W43A .
We compare the magnetic field obtained in the OH maser region with the one measured in the H _ { 2 } O maser jet .

Methods : We used the UK Multi-Element Radio Linked Interferometer Network ( MERLIN ) to observe the polarization of the OH masers in the circumstellar envelope of W43A .
Likewise , we used the Green Bank Telescope ( GBT ) observations to measure the magnetic field strength obtained previously in the H _ { 2 } O maser jet .

Results : We report a measured magnetic field of approximately 100 \mu G in the OH maser region of the circumstellar envelope around W43A .
The GBT observations reveal a magnetic field strength B _ { || } of \sim 30 mG changing sign across the H _ { 2 } O masers at the tip of the red-shifted lobe of the bipolar outflow .
We also find that the OH maser shell shows no sign of non-spherical expansion and that it probably has an expansion velocity that is typical for the shells of regular OH/IR stars .

Conclusions : The GBT observations confirm that the magnetic field collimates the H _ { 2 } O maser jet , while the OH maser observations show that a strong large scale magnetic field is present in the envelope surrounding the W43A central star .
The magnetic field in the OH maser envelope is consistent with the one extrapolated from the H _ { 2 } O measurements , confirming that magnetic fields play an important role in the entire circumstellar environment of W43A .