of H2O maser polarization

Context. A low- or intermediate-mass star is believed to maintain a spherical shape throughout the evolution from the main sequence to the asymptotic giant branch (AGB) phase. However, many post-AGB objects and planetary nebulae exhibit non-spherical symme-try. Several candidates have been suggested as factors that can play a role in this change of morphology, but the problem is still not well understood. Magnetic fields are one of these possible agents. Aims. We aim to detect the magnetic field and infer its properties around four AGB stars using H2O maser observations. The sam-ple we observed consists of the following sources: the semi-regular variable RT Vir, and the Mira variables AP Lyn, IK Tau, and IRC+60370. Methods. We observed the 61,6−52,3 H2O maser rotational transition in full-polarization mode to determine its linear and circular polarization. Based on the Zeeman effect, one can infer the properties of the magnetic field from the maser polarization analysis. Results. We detected a total of 238 maser features in three of the four observed sources. No masers were found toward AP Lyn. The observed masers are all located between 2.4 and 53.0 AU from the stars. Linear and circular polarization was found in 18 and 11 maser features, respectively. Conclusions. We more than doubled the number of AGB stars in which a magnetic field has been detected from H2O maser polar