The Compton Gamma Ray Observatory (CGRO) satellite has recently discovered that
highly variable and radio-loud quasars are also strong 
-ray emitters
(Fichtel et al. 1994; von Montigny et al.
1995; Thompson et al. 1995). The
power released by such objects above 
can overcome that in the
other bands by even two orders of magnitude (see e.g. Dondi & Ghisellini
1995).
Many theoretical models have been proposed in order to explain this phenomenon
and observational constraints are the only tool that can operate a choice among
them. According to a group of models, the 
-ray emission can be due to
inverse Compton scattering of optical photons on relativistic electrons inside
a jet. In turn, the seed photons may either be produced by the same
relativistic electrons through synchrotron process
(Maraschi et al. 1992) or
come from some external region [e.g. from the accretion disk, as in the model
by Dermer & Schlickeiser (1993), or from the broad line region illuminated by
the accretion disk, as in the model by Sikora et al. (1994)].
Through the optical observation of strong 
-ray emitting blazars it is
thus possible to obtain information about the mechanisms that rule the
production of the seed photons for the 
radiation. In particular,
optical monitoring simultaneous with 
observations can confirm the
expected temporal correlation between the two emissions and constitutes a
crucial test for the theoretical predictions. Indeed, in the case of seed
photons produced by the same relativistic electrons which are responsible for
the 
radiation, besides a simultaneous variability in the two bands,
larger amplitude variations in the 
one are expected, since any
increase in the relativistic electron number contributes twice to the 
emission. If, on the contrary, the seed photons were produced outside, the
-ray emission intensity would vary more or less linearly with the
optical one. A non-simultaneous variability would be a problem for both the
models.
For the above reasons, an optical monitoring campaign has been started at the
Torino Observatory since November 1994 with the aim of following the optical
behaviour of a list of 
-ray loud blazars. The observations were
intensified, when possible, during CGRO pointings. Collaborations with other
institutes were started on some sources: as for OJ 287, 3C 66A, and AO 0235+164
we joined the OJ-94 Project (e.g. Sillanpää et al.
1996; Takalo et al.
1996a,b; Villata et al. 1996), while for OF 038, S5 0716+714, PKS 0735+178, S4
0954+658, and ON 231 a national collaboration with the Roma and Perugia groups
was set up (Massaro et al. 1996; Ghisellini
et al. 1996; Latini et al. 1996;
Fiorucci et al. 1996; Tosti et al.
1996). In this paper we present the
results of the optical monitoring of the remaining twenty blazars in the Torino
program list. In Sect. 2 a description of the instrumental equipment and data
reduction procedure is given and the list of the observed sources is presented.
Sect. 3 contains, for each source, a brief summary of previously published
data in the optical (and 
) band and the presentation and description of
the light curves we obtained within the period from November 15, 1994 to
November 7, 1995. For the BL Lac object PKS 2254+074 we show the results of
UBVRI photopolarimetry too. Photometric sequence calibrations are also given
for ten sources. Finally, the main conclusions of this work are drawn in Sect.
4.