GRAVITY Astrometry of the Young Triple Star TWA5

The VLTI instrument GRAVITY was designed to measure not only visibilities, but also astrometry with micro-arcsecond precision. This opens new possibilities for orbit and mass determinations of young multiple stars.
One special application are triple systems, where we can determine the separation, position angle, and flux ratio of the inner binary from its visibilities, while simultaneously measuring its position relative to the third component.  This way, we can determine the individual orbits and masses of the binary components, not just the sum of their masses.
The method was used for the first time in our observations of the young triple star TWA5.
The data reduction is still experimental since the astrometric mode is only partially supported by the ESO pipeline.  We describe our approach to reduce the data and present first results of this project.