  Magnetic activity on stars manifests itself in the form of dark spots on the
stellar surface, that cause modulation of a few percent in the light curve of
the star as it rotates. When a planet eclipses its host star, it might cross in
front of one of these spots creating a "bump" in the transit light curve. By
modelling these spot signatures, it is possible to determine the physical
properties of the spots such as size, temperature, and location. In turn, the
monitoring of the spots longitude provides estimates of the stellar rotation
and differential rotation. This technique was applied to the star Kepler-17, a
solar--type star orbited by a hot Jupiter. The model yields the following spot
characteristics: average radius of $49 \pm 10$ Mm, temperatures of $5100 \pm
300$ K, and surface area coverage of $6 \pm 4$ \%. The rotation period at the
transit latitude, $-5^\circ$, occulted by the planet was found to be $11.92 \pm
0.05$ d, slightly smaller than the out--of--transit average period of $12.4 \pm
0.1$ d. Adopting a solar like differential rotation, we estimated the
differential rotation of Kepler-17 to be $\Delta\Omega = 0.041 \pm 0.005$ rd/d,
which is close to the solar value of 0.050 rd/d, and a relative differential
rotation of $\Delta\Omega/\Omega=8.0 \pm 0.9$ \%. Since Kepler-17 is much more
active than our Sun, it appears that for this star larger rotation rate is more
effective in the generation of magnetic fields than shear.
