  Many planet candidates have been detected by radial velocity variations of
the primary star; they are planet candidates, because of the unknown orbit
inclination. Detection of the wobble in the two other dimensions, to be
measured by astrometry, would yield the inclination and, hence, true mass of
the companions. We aim to show that planets can be confirmed or discovered in a
close visual stellar binary system by measuring the astrometric wobble of the
exoplanet host star as periodic variation of the separation, even from the
ground. We test the feasibility with HD 19994, a visual binary with one radial
velocity planet candidate. We use the adaptive optics camera NACO at the VLT
with its smallest pixel scale (13 mas) for high precision astrometric
measurements. The separations measured in single 120 images taken within one
night are shown to follow white noise, so that the standard deviation can be
devided by the square root of the number of images to obtain the precision. In
this paper we present the first results and investigate the achievable
precision in relative astrometry with adaptive optics. With a careful data
reduction it is possible to achieve a relative astrometric precision as low as
50 micro arc sec for a 0.6 arc sec, binary with VLT/NACO observations in one
hour, the best relative astrometric precision ever achieved with a single
telescope from the ground. The relative astrometric precision demonstrated here
with AO at an 8-m mirror is sufficient to detect the astrometric signal of the
planet HD 19994 Ab as periodic variation of the separation between HD 19994 A
and B.
