  The non-thermal 3.6 cm radio continuum emission from the young stars
  S1 and DoAr21 in the core of Ophiuchus, has been observed with the
  Very Long Baseline Array (VLBA) at 6 and 7 epochs, respectively, between June
2005 and August 2006. The typical separation between successive observations
was 2 to 3 months. Thanks to the remarkably accurate astrometry delivered by
the VLBA, the trajectory described by both stars on the plane of the sky could
be traced very precisely, and modeled as the superposition of their
trigonometric parallax and a uniform proper motion. The best fits yield
distances to S1 and DoAr21 of 116.9$^{+7.2}_{-6.4}$ pc and
121.9$^{+5.8}_{-5.3}$ pc, respectively. Combining these results, we estimate
the mean distance to the Ophiuchus core to be 120.0$^{+4.5}_{-4.2}$ pc, a value
consistent with several recent indirect determinations, but with a
significantly improved accuracy of 4%. Both S1 and DoAr21 happen to be members
of tight binary systems, but our observations are not frequent enough to
properly derive the corresponding orbital parameters. This could be done with
additional data, however, and would result in a significantly improved accuracy
on the distance determination.
