  We present our search program for substellar companions using high-precision
relative astronomy. Due to its orbital motion around the star, an unseen
substellar companion would produce a periodic "wobble" of the host star, which
is the astrometric signal of the unseen companion. By measuring the separation
between the components of stellar double and triple systems, we want to measure
this astrometric signal of a possible unseen companion indirectly as a relative
and periodic change of these separations. Using a new observation mode (the
"cube-mode") where the frames were directly saved in cubes with nearly no loss
of time during the readout, an adaptive optics system to correct for
atmospheric noise and an infrared narrow band filter in the near infrared to
suppress differential chromatic refraction (DCR) effects we achive for our
first target (the double star HD 19994) a relative precision for the separation
measurements of about 100...150 micro-arsecond per epoch. To reach a precision
in the micro-arcsec-regime, we use a statistical approach. We take several
thousand frames per target and epoche and after a verification of a Gaussian
distribution the measurement precision can be calculated as the standard
deviation of our measurements divided by the square root of the number of
Gaussian distributed measurements. Our first observed target is the stellar
binary HD 19994 A & B, where the A component has a known radial velocity planet
candidate.
