Atmospheric fluctuations and hydrogen maser clock instabilities limit the coherence time (Tcoh) and hence the signal-to-noise ratio of VLBI data , to a few minutes at GHz frequencies. However, Tcoh can be increased up to many hours if a phase reference is established throughout the experiment by using a strong source observed alternatively with the program source. Weak celestial radio sources with flux densities in the milliJansky range at centimeter wavelengths, such as radio stars, fast pulsars and faint components of extragalactic sources, can then be reliably detected by this VLBI technique, simultaneously providing accurate astrometry relative to the reference source.