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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 and the sky-projected orbital obliquity of the system can then be determined. If one has three or more observations of the same spot, the true orbital obliquity 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 sky-projected and true orbital obliquity 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.

Poster presented at Cool Stars 19, Uppsala, June 2016