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CoRoT-3b is a short-period transiting companion with roughly $22$ Jupiter masses but a radius close to Jupiter's. Its discovery combined a sequence of $34$ exoplanet transits with ground-based Doppler spectroscopy and additional checks. This measured both a large mass and a compact radius, placing it above the ordinary giant-planet population in mean mass density.^[ref]

The object illustrates why a measured radius alone does not distinguish a giant planet from a brown dwarf: electron degeneracy pressure can give very different masses similar radii. Its mass also does not uniquely reveal its formation mechanism. The discovery paper explicitly left the formation-based interpretation open between a low-mass brown dwarf and an unusually massive planetary companion.

Magali Deleuil coauthored a CoRoT transit study of a high-mass close-orbiting companion. Such measurements connect exoplanet transit photometry, radial velocities and structural models, testing the boundary between commonly used planetary and substellar classifications.^[ref]