  The origin of magnetism around AGB stars is uncertain. If these stars drive a
dynamo, it cannot be sufficient to generate a strong global field, otherwise
the observed X-ray luminosities would be higher. Other explanations for the
circular polarization of SiO masers in the atmospheres are needed. The
interaction of the AGB wind with previously ejected matter and with planets
bears complex magneto-hydrodynamic phenomena on a short time scale, such that
strong magnetic fields can be maintained locally. Here we provide observational
evidence for the corresponding magnetic fluctuations. We trace magnetic
activity with the circular polarization of the v=1, J=2-1 SiO masers, using a
correlation polarimeter. In V Cam and R Leo, we find evidence of
pseudo-periodic fluctuations of the circular polarization on a timescale of a
few hours, from which we infer magnetic fluctuations of ~1G. The phenomenon is
rare and restricted to a narrow range of velocities. It seems to be associated
with planetary wake flows suggested by VLBI maps. While scenarios involving
magnetic activity in the extended stellar atmosphere have problems to explain
all observed features, precessing Jovian magnetospheres predict all of them.
For the case of R Leo, we constrain the orbit of the planet (estimated period
5.2 years) and estimate a stellar mass of 0.7 solar masses.
