  Hubble Space Telescope (HST) observations provide a novel way of testing
unified models for FR I radio sources and BL Lac objects. The detection of
extended dust discs in some radio galaxies provides information on their jet
orientation. Given this, the strength of the compact nuclear sources of FR I
and BL Lacs can be compared with model predictions. As a pilot project, we
selected five radio galaxies which show extended nuclear discs in the HST
images. The relative orientation of the projected radio-jets and of the
extended nuclear discs indicates that they are not perpendicular, as the
simplest geometrical model would suggest, but that they form an angle of ~ 20 -
40 degrees with the symmetry axis of the disc: a significant change of
orientation occurs between the innermost AGN structure and the kpc-scale.
Nevertheless, the discs appear to be useful indicators of the radio sources
orientation since the angles formed by the disc axis and the jet with the line
of sight differ by only ~ 10 - 20 degrees. At the center of each disc an
unresolved nuclear source is present. We compared its luminosity with the
optical core luminosity of BL Lacs selected for having similar host galaxy
magnitude and extended radio luminosity. The BL Lac cores are between 2 E2 and
3 E5 times brighter than the corresponding radio galaxies ones. The FR I/BL Lac
core luminosity ratio shows a suggestive correlation with the orientation of
the radio galaxies with respect to the line of sight. The behavior of this
ratio is quantitatively consistent with a scenario in which the emission in the
FR I and BL Lac is dominated by the beamed radiation from a relativistic jet
with Doppler factor ~ 5 - 10, thus supporting the basic features of the
proposed unification schemes.
