  Eclipsing time variations have been observed for a wide range of binary
systems, including post-common-envelope binaries. A frequently proposed
explanation, apart from the possibility of having a third body, is the effect
of magnetic activity, which may alter the internal structure of the secondary
star, particularly its quadrupole moment, and thereby cause quasi-periodic
oscillations. Here we present two compressible non-ideal magneto-hydrodynamical
(MHD) simulations of the magnetic dynamo in a solar mass star, one of them with
three times the solar rotation rate ("slow rotator"), the other one with twenty
times the solar rotation rate ("rapid rotator"), to account for the high
rotational velocities in close binary systems. For the slow rotator, we find
that both the magnetic field and the stellar quadrupole moment change in a
quasi-periodic manner, leading to O-C (observed - corrected times of the
eclipse) variations of ~0.025 s. For the rapid rotator, the behavior of the
magnetic field as well as the quadrupole moment changes become considerably
more complex, due to the less coherent dynamo solution. The resulting O-C
variations are of the order 0.13 s. The observed system V471~Tau shows two
modes of eclipsing time variations, with amplitudes of 151 s and 20 s,
respectively. However, the current simulations may not capture all relevant
effects due to the neglect of the centrifugal force and self-gravity.
Considering the model limitations and that the rotation of V471 Tau is still a
factor of 2.5 faster than our rapid rotator, it may be conceivable to reach the
observed magnitudes.
