Analytical predictions for the astrometric effect of a moon with varying mass and distance to HD 206893 B. GRAVITY could in principle detect moons lower than a Neptune mass at larger periods. The fit of the GRAVITY data is shown as a green point assuming the mass and period as upper limits in our tentative detection. The orange area shows the plausible parameter space where a moon might be located, and still be potentially detectable with GRAVITY assuming an astrometric accuracy of 10 μas (see Sect. 3.3).