  Combining adaptive optics and interferometric observations results in a
considerable contrast gain compared to single-telescope, extreme AO systems.
Taking advantage of this, the ExoGRAVITY project is a survey of known young
giant exoplanets located in the range of 0.1'' to 2'' from their stars. The
observations provide astrometric data of unprecedented accuracy, being crucial
for refining the orbital parameters of planets and illuminating their dynamical
histories. Furthermore, GRAVITY will measure non-Keplerian perturbations due to
planet-planet interactions in multi-planet systems and measure dynamical
masses. Over time, repetitive observations of the exoplanets at medium
resolution ($R=500$) will provide a catalogue of K-band spectra of
unprecedented quality, for a number of exoplanets. The K-band has the unique
properties that it contains many molecular signatures (CO, H$_2$O, CH$_4$,
CO$_2$). This allows constraining precisely surface gravity, metallicity, and
temperature, if used in conjunction with self-consistent models like Exo-REM.
Further, we will use the parameter-retrieval algorithm petitRADTRANS to
constrain the C/O ratio of the planets. Ultimately, we plan to produce the
first C/O survey of exoplanets, kick-starting the difficult process of linking
planetary formation with measured atomic abundances.
