  Stellar activity features such as spots can create complications in
determining planetary parameters through spectroscopic and photometric
observations. The overlap of a transiting planet and a stellar spot, for
instance, can produce anomalies in the transit light curves that may lead to
inaccurate estimation of the transit duration, depth and timing. For instance
such inaccuracies can affect the precise derivation of the planet's radius. In
this paper, we present the results of a quantitative study on the effects of
stellar spots on high precision transit light curves. We show that spot
anomalies can lead to the estimate of a planet radius that is 4% smaller than
the real value. The effects on the transit duration can also be of the order of
4%, longer or shorter. Depending on the size and distribution of spots,
anomalies can also produce transit timing variations with significant
amplitudes. For instance, TTVs with signal amplitudes of 200 seconds can be
produced when the spot is completely dark and as large as the largest Sun spot.
Our study also indicates that the minimum size of a stellar spot with
detectable affects on the high precision transit light curve is around 0.03
time the stellar radius for typical Kepler telescope precision. We also show
that the strategy of including more free parameters (such as transit depth and
duration) in the fitting procedure to measure the transit time of each
individual transit will not produce accurate results in the case of active
stars.
