  A new method for determining the stellar rotation period is proposed here,
based on the detection of starspots during transits of an extra-solar planet
orbiting its host star. As the planet eclipses the star, it may pass in front
of a starspot which will then make itself known through small flux variations
in the transit light curve. If we are lucky enough to catch the same spot on
two consecutive transits, it is possible to estimate the stellar rotational
period. This method is successfully tested on transit simulations on the Sun
yielding the correct value for the solar period. By detecting two starspots on
more than one transit of HD 209458 observed by the Hubble Space Telescope, it
was possible to estimate a period of either 9.9 or 11.4 days for the star,
depending on which spot is responsible for the signature in the light curve a
few transits later. Comparison with period estimates of HD209458 reported in
the literature indicates that 11.4 days is the most likely stellar rotation
period.
