Kinematics of the Galaxy from OB Stars with Data from the Gaia DR2
  Catalogue
  We have selected and analyzed a sample of OB stars with known line-of-sight
velocities determined through ground-based observations and with trigonometric
parallaxes and proper motions from the Gaia DR2 catalogue. Some of the stars in
our sample have distance estimates made from calcium lines. A direct comparison
with the trigonometric distance scale has shown that the calcium distance scale
should be reduced by 13\%. The following parameters of the Galactic rotation
curve have been determined from 495 OB stars with relative parallax errors less
than 30\%:
  $(U,V,W)_\odot=(8.16,11.19,8.55)\pm(0.48,0.56,0.48)$ km s$^{-1}$,
  $\Omega_0=28.92\pm0.39$ km s$^{-1}$ kpc$^{-1}$,
  $\Omega^{'}_0=-4.087\pm0.083$ km s$^{-1}$ kpc$^{-2}$ and
  $\Omega^{''}_0=0.703\pm0.067$ km s$^{-1}$ kpc$^{-3}$, where the circular
velocity of the local standard of rest is
  $V_0=231\pm5$ km s$^{-1}$ (for the adopted $R_0=8.0\pm0.15$ kpc). The
parameters of the Galactic spiral density wave have been found from the series
of radial, $V_R,$ residual tangential, $\Delta V_{circ}$, and vertical, $W,$
velocities of OB stars by applying a periodogram analysis. The amplitudes of
the radial, tangential, and vertical velocity perturbations are
  $f_R=7.1\pm0.3$ km s$^{-1}$,
  $f_\theta=6.5\pm0.4$ km s$^{-1}$, and
  $f_W=4.8\pm0.8$ km s$^{-1}$, respectively; the perturbation wavelengths are
  $ \lambda_R=3.3\pm0.1$ kpc,
  $\lambda_\theta=2.3\pm0.2$ kpc, and
  $ \lambda_W=2.6\pm0.5$ kpc; and the Sun's radial phase in the spiral density
wave is
  $ (\chi_\odot)_R=-135\pm5^\circ$,
  $(\chi_\odot)_\theta=-123\pm8^\circ$, and
  $ (\chi_\odot)_W=-132\pm21^\circ$ for the adopted four-armed spiral pattern.
