Blazars are known to show, besides variability on long periods, flux changes on
small time scales, i.e. shorter than one day. This microvariability has been
observed in many bands, from radio to 
-rays (see the review paper by
Wagner & Witzel (1995)), and implies emission from regions whose size is
comparable to that of our Solar System. The mechanisms responsible for such
rapid variations are still unclear, different interpretations being possible,
and much observational effort is needed to understand this point.
In this section we present three cases of intranight variability that we were able to detect for 3C 66A and OJ 287. Since these sources are the subjects of the OJ-94 Project, we refer to the relative papers for the general behaviour of their light curves (Sillanpää et al. 1996a,b; Takalo et al. 1996). Magnitude calibration was performed according to Villata et al. (1996).
In Fig. 21 (click here) the light curve in the R band of 3C 66A during the night of August 29-30, 1995 is shown.
Figure 21: Light curve of 3C 66A in the R band during the
night of August 29-30,
1995
A well defined trend is evident, with a total
brightness increase of 
in 6 hours and 26 minutes. The global
increasing rate is 
per hour, while the increasing rate of the
final, steeper part is 
per hour. This value is comparable to
the decreasing rate of 
per hour detected during the night of
October 24-25, 1995, when the source acquired 
in
about 6 hours (see Fig. 22 (click here)).
Figure 22: Light curve of 3C 66A in the R band during the night of October 24-25,
1995
The microvariability characteristics of OJ 287 is the subject of a recent work
by González-Pérez et al. (1996), covering 26 nights from
November 1993 to
November 1994. They found variations in 
of the nights.
Figure 23: Light curve of OJ 287 in the R band during the night of December 19-20,
1995
In their study
on optical intraday variability of radio selected BL Lac objects, Heidt &
Wagner (1996) found a characteristic time scale of 2.8 days for OJ 287, based
on a structure function analysis, and a variability amplitude of 
.
The intranight behaviour of the OJ 287 R magnitude in the night of December
19-20, 1995 is plotted in Fig. 23 (click here).
One can see a fast brightening of
in one hour, after which the source magnitude remained more or
less stable. A similar rate of magnitude variation during another night can be
found in Sillanpää et al. (1996c).