  Context. Nulling interferometry has been suggested as the underlying
principle for the Darwin and TPF-I exoplanet research missions. Aims. There are
constraints both on the mean value of the nulling ratio, and on its stability.
Instrument instability noise is most detrimental to the stability of the
nulling performance. Methods. We applied a modified version of the classical
dithering technique to the optical path difference in the scientific beam.
Results. Using only this method, we repeatedly stabilised the dark fringe for
several hours. This method alone sufficed to remove the 1/ f component of the
noise in our setup for periods of 10 minutes, typically. These results indicate
that performance stability may be maintained throughout the long-duration data
acquisitions typical of exoplanet spectroscopy. Conclusions. We suggest that
further study of possible stabilisation strategies should be an integral part
of Darwin/TPF-I research and development
