  We report the discovery and characterisation of OGLE-TR-122b, the smallest
main-sequence star to date with a direct radius determination. OGLE-TR-122b
transits around its solar-type primary every 7.3-days. With M=0.092+-0.009 Mo
and R=0.120 +0.024-0.013 Ro, it is by far the smallest known eclipsing M-dwarf.
The derived mass and radius for OGLE-TR-122b are in agreement with the
theoretical expectations. OGLE-TR-122b is the first observational evidence that
stars can indeed have radii comparable or even smaller than giant planets. In
such cases, the photometric signal is exactly that of a transiting planet and
the true nature of the companion can only be determined with high-resolution
spectroscopy.
