Existing upper limits on variations in the photospheric radius of the Sun during the solar magnetic activity cycle are at a fractional amplitude of \sim 2 \times 10 ^ { -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 .