Applegate mechanism for Hot Jupiters

...or how to complicate transit timing variations analysis.

Orbital period variations of hot-Jupiters caused by the Applegate effect

Stellar activity changes the quadrupole moment of the star, leading to orbital variations of the planet. Could be mistaken for influence of an additional planet.

Similar effects are seen in binary star systems: note the discovery papers for circumbinary planets - at least the free-access ones - do try to analyse whether the timing variations could be down to the Applegate mechanism.