Kinematics of OB-associations in Gaia epoch
  We use stellar proper motions from the TGAS catalog to study the kinematics
of OB-associations. The TGAS proper motions of OB-associations generally agree
well with the Hipparcos proper motions. The parameters of the Galactic rotation
curve obtained with TGAS and Hipparcos proper motions agree within the errors.
The average one-dimensional velocity dispersion inside 18 OB-associations with
more than 10 TGAS stars is sigma_v=3.9 km s-1, which is considerably smaller,
by a factor of 0.4, than the velocity dispersions derived from Hipparcos data.
The effective contribution from orbital motions of binary OB-stars into the
velocity dispersion sigma_v inside OB-associations is sigma_b=1.2 km s-1. The
median virial and stellar masses of OB-associations are equal to 7.1 10^5 and
9.0 10^3 Ms, respectively. Thus OB-associations must be unbound objects
provided they do not include a lot of dense gas. The median star-formation
efficiency is epsilon=2.1 percent. Nearly one third of stars of OB-associations
must lie outside their tidal radius. We found that the Per OB1 and Car OB1
associations are expanding with the expansion started in a small region of
11--27 pc 7--10 Myr ago. The average expansion velocity is 6.3 km s-1.
