Long-Term Evolution in Transit Duration of Extrasolar Planets from   Magnetic Activity in their Parent Stars
  Existing upper limits on variations in the photospheric radius of the Sun during the solar magnetic activity cycle are at a fractional amplitude of 2x10^{-4}. At that level, the transit duration of a close-in planet around a Sun-like star could change by a fraction of a second per year. This magnitude of variation is larger than that caused by other studied effects (owing to proper motion or general-relativistic effects), and should be included in the analysis of constraints on multi-planet systems from transit timing. A temporal correlation between the transit duration and spectroscopic measures of stellar activity can be used to separate the stellar radius change from other effects. The magnetic activity effect could be significantly larger for late-type stars, such as M-dwarfs, which are more variable than the Sun. In general, precision transit measurements provide a new tool for measuring long-term variations of stellar radii. 
Abraham Loeb (Harvard)