  When a planet passes in front of a starspot during a transit of its host
star, it causes a small upward blip in the light curve. Modelling the transit
with the starspot allows the size, brightness and position of the spot to be
measured. If the same spot can be observed in two different transits, it is
possible to track the motion of the spot due to the rotation of the star. The
rotation period and velocity of the star (Prot and Vsini) and the sky-projected
orbital obliquity of the system (lambda) can then be determined. If one has
three or more observations of the same spot, the true orbital obliquity (psi)
can be measured. We are performing this analysis for a number of cool stars
orbited by transiting planets. We present our results so far and compile a
catalogue of lambda and psi measurements from spot crossing events. The method
is particularly useful for cool stars, and is therefore complementary to
studies of the Rossiter-McLaughlin effect, which perform better on hotter and
faster-rotating stars.
