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The observed eclipsing time variations in post-common-envelope binaries(PCEBs) can be interpreted as potential evidence for massive Jupiter-likeplanets, or as a result of magnetic activity, leading to quasi-periodic changesin the quadrupole moment of the secondary star. The latter is commonly referredto as the Applegate mechanism. Following Brinkworth et al. (2006), we employhere an improved version of Applegate's model including the angular momentumexchange between a finite shell and the core of the star. The framework isemployed to derive the general conditions under which the Applegate mechanismcan work, and is subsequently applied to a sample of 16 close binary systemswith potential planets, including 11 PCEBs. Further, we present a detailedderivation and study of analytical models which allow for an straightforwardextension to other systems. Using our full numerical framework, we show thatthe Applegate mechanism can clearly explain the observed eclipsing timevariations in 4 of the systems, while the required energy to produce thequadrupole moment variations is too high in at least 8 systems. In theremaining 4 systems, the required energy is comparable to the available energyproduced by the star, which we consider as borderline cases. Therefore, theApplegate mechanism cannot uniquely explain the observed period time variationsfor this entire population. Even in systems where the required energy is toohigh, the Applegate mechanism may provide an additional scatter, which needs tobe considered in the derivation and analysis of planetary models.

Eclipsing time variations in close binary systems: Planetary hypothesis vs. Applegate mechanism