  Context: To investigate the differences between positions, as determined by
optical (direct imaging) and Very Long Baseline Interferometry (VLBI)
techniques, of extragalactic sources listed in the second realization of the
International Celestial Reference Frame (ICRF2). Aims: To verify the influence
of the source's intrinsic structure on these differences. Methods: Instruments
with mosaics of CCDs were used to acquire the optical positions presented here,
leading us to opt for overlapping techniques to build a virtual, continuous CCD
over the whole angular size of the respective fields of view, whose translation
of the resulting intrumental positions into positions that are consistent with
those in the ICRF2 was made with the help of the UCAC2. Results: The
differences obtained between the optical and VLBI positions of the observed
sources may reach more than 80 milliarcseconds and, taking into consideration
that they are hardly explained only by statistical fluctuations or systematic
errors in the optical reference frame used here, we argue that these
differences can be related to the sources' X-band structure index (8.4 GHz).
Conclusions: In this context, the presence of the intrinsic structure should be
taken into consideration when comparing the optical and VLBI positions of ICRF2
sources in the future.
