  Imaging Atmospheric Cherenkov Telescopes (IACTs) currently in operation
feature large mirrors and order of 1 ns time response to signals of a few
photo-electrons produced by optical photons. This means that they are ideally
suited for optical interferometry observations. Thanks to their sensitivity to
visible wavelengths and long baselines optical intensity interferometry with
IACTs allows reaching angular resolutions of tens to microarcsec. We have
installed a simple optical setup on top of the cameras of the two 17 m diameter
MAGIC IACTs and observed coherent fluctuations in the photon intensity measured
at the two telescopes for three different stars. The sensitivity is roughly 10
times better than that achieved in the 1970s with the Narrabri interferometer.
