  Unresolved nuclear sources are detected by the Hubble Space Telescope in the
great majority of a complete sample of 33 FR I radio galaxies belonging to the
3CR catalogue.
  The optical flux of these Central Compact Cores (CCC) shows a striking linear
correlation with the radio core one over four decades, arguing for a
non--thermal synchrotron origin of the CCC radiation. We also find evidence
that this emission is anisotropic, which leads us to identify CCCs with the
misoriented relativistic jet component which dominates in BL Lac objects. This
interpretation is also supported by the similarity in the radio-to-optical and
optical spectral indices.
  The high rate of CCC detection (85 %) suggests that a `standard' pc scale,
geometrically thick torus is not present in low luminosity radio-galaxies. Thus
the lack of broad lines in FR I cannot be attributed to obscuration.
  CCC fluxes also represent upper limits to any thermal/disc emission. For a
$10^9 M_{\sun}$ black hole, typical of FR I sources, these limits translate
into a fraction as small as less than $10^{-7}-10^{-5}$ of the Eddington
luminosity.
