  As part of an astrometric program, we have used the Very Long Baseline Array
to measure the trigonometric parallax of several young stars in the Taurus and
Ophiuchus star-forming regions with great accuracy. Additionally, we have
obtained an unprecedented sample of high-resolution (~ 1 mas) images of several
young stellar systems. These images revealed that about 70% of the stars in our
sample are very tight binary stars (with separations of a few mas). Since it is
highly unlikely that 70% of all stars are such tight binaries, we argue that
selection effects are at work.
