Distance scale for high-luminosity stars in OB associations and in field
  with Gaia DR2. Spurious systematic motions
  We calculated the median parallaxes for 47 OB associations including at least
10 stars with known Gaia DR2 parallaxes. A comparison between trigonometric and
photometric parallaxes of OB associations reveals a zero-point offset of
delta_pi=-0.11 +\- 0.04 mas indicating that Gaia DR2 parallaxes are, on
average, underestimated and the distances derived from them are overestimated.
The correction of delta_pi=-0.11 mas is consistent with the estimate that
Arenou et al. (2018) obtained for bright stars. An analysis of parallaxes of OB
associations and high-luminosity field stars confirms our previous conclusion
(Dambis et al. 2001) that the distance scale for OB stars established by Blaha
and Humphreys (1989) must be reduced by 10--20%. Spurious systematic motions of
10--20 km s-1 at the distances of 2--3 kpc from the Sun are found to arise from
the use of the uncorrected Gaia DR2 parallaxes.
