Parameters of the Link between the Optical and Radio Frames from Gaia
  DR2 Data and VLBI Measurements
  Based on published data, we have assembled a sample of 88 radio stars for
which there are both trigonometric parallax and proper motion measurements in
the Gaia DR2 catalogue and VLBI measurements. A new estimate of the systematic
offset between the optical and radio frames has been obtained by analyzing the
GaiaDR2-VLBI trigonometric parallax differences: $\Delta\pi=-0.038\pm0.046$ mas
(with a dispersion of 0.156 mas). This means that the Gaia DR2 parallaxes
should be increased by this correction. The parallax scale factor is shown to
be always very close to unity within $\sim$3 kpc of the Sun: $b=1.002\pm0.007.$
Our analysis of the proper motion differences for the radio stars based on the
model of solid-body mutual rotation has revealed no rotation components
differing significantly from zero:
$(\omega_x,\omega_y,\omega_z)=(-0.14,0.03,-0.33)\pm(0.15,0.22,0.16)$ mas
yr$^{-1}.$
