Context: Planetary nebulae (PNe) often show large departures from
spherical symmetry. The origin and development of these asymmetries is not clearly
understood. The most striking structures are the highly collimated jets that are already
observed in a number of evolved stars before they enter the PN phase.

Aim: The aim of this project is to observe the Zeeman splitting of the OH maser
of the W43A star and determine the magnetic field strength in the low density region.
Methods: The 1612 MHz OH masers of W43A were observed with MERLIN to measure
the circular polarization due to the Zeeman splitting of 1612 OH
masers in the envelope of the evolved star W43A.
Results: We measured the circular polarization, possibly due to the Zeeman
effect, of the strongest 1612 OH maser of W43A and found a magnetic field strength of 59±4 μG.
The field strength is consistent with extrapolated water maser measurements, however, the
interpretation of the circular polarization is ambiguous, as the Zeeman splitting is much less
than the maser line width.
Conclusions: If the circular polarization is indeed the result of Zeeman splitting,
the measured magnetic field further confirms that the large scale magnetic field
is present in W43A, which likely plays a role in collimating the jet.