  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.
