Within the binary stars zoo, we can find the post-common-envelope binaries (PCEBs) which are commonly composed of a magnetically active, late type star, and a white dwarf. These are close binaries that have undergone a common envelope phase in their past and now have binary separations as small as one solar radius with orbital periods of just a few hours. Observations show that approximately 90 percent of the eclipsing times of PCEBs have semi-periodic variations. One interpretation of such phenomenon is that there are circumbinary planets or brown dwarfs that affect the barycenter. Another possibility is given by variations of the gravitational quadrupole moment (Q) of the active star. Increasing Q means that the oblateness of the star is increased and therefore the binary separation is reduced.
In this talk I will discuss the second possibility by means of self-consistent three-dimensional magneto-hydrodynamical simulations. I will discuss our results in the context of variations of Q due to (1) magnetic fields only, (2) magnetic fields with the centrifugal force, know as the Applegate mechanism, and (3) non-axisymmetric Q, now known as the Applegate-Lanza mechanism.
