High-precision angular diameter of V3879 Sgr

We use commissioning data obtained on the bright single star V3879 Sgr. Locking the interferometric fringes with FINITO allows us to record very low contrast fringes on the AMBER camera.

By fitting the amplitude of these fringes, we measure the diameter of the target in three directions simultaneously with an accuracy of 25micro-arcseconds. We showed that V3879 Sgr has a round photosphere down to a sub-percent level. We quickly reached this level of accuracy because the technique used is independent from absolute calibration (at least for baselines that fully span the visibility null).