In the following figures only light curves in the R band (in general the best sampled ones) are shown. Pointing periods of the EGRET instrument on board CGRO are indicated by boxes. In some cases the data published in Villata et al. (1997a) relative to the first year monitoring are presented together with the new data in order to better describe the source behaviour. In other cases only the latest data are shown in the light curves to make the plot clearer. In any case, tables contain all observations made by our group in the R, V, and B bands from November 1994, together with indication of the minimum and maximum magnitudes registered and the number of frames taken in each band.
In subsets to the light curve figures an estimate of the measure accuracy is given by plotting the deviation from the mean magnitude difference between two reference stars. These deviations are also shown as error bars on the corresponding points of the light curve. However, lower limits on the error bars are set, taking also into account the brightness difference between the source and the reference stars: the nearer the brightness, the more reliable the error estimate given by the above deviation.
Table 1 (click here) contains the list of our sources, with indication of their name, right
ascension, and declination. A brief outline of the observational material in
the optical and 
bands available in the literature was already
presented in Villata et al. (1997a), so that in this paper
we quote the most recent papers only.
| Source | Name | RA (2000) | Dec. (2000) |
| 0219+428 | 3C 66A | 02 22 39.61 | +43 02 07.8 |
| 0420-014 | PKS 0420-01 | 04 23 15.80 | -01 20 33.1 |
| 0422+004 | PKS 0422+00 | 04 24 46.84 | +00 36 06.3 |
| 0528+134 | PKS 0528+134 | 05 30 56.42 | +13 31 55.1 |
| 0805-077 | PKS 0805-07 | 08 08 15.54 | -07 51 09.9 |
| 0827+243 | OJ 248 | 08 30 52.09 | +24 10 59.8 |
| 0836+710 | 4C 71.07 | 08 41 24.37 | +70 53 42.2 |
| 0851+202 | OJ 287 | 08 54 48.87 | +20 06 30.6 |
| 0906+430 | 3C 216 | 09 09 33.50 | +42 53 46.1 |
| 1101+384 | Mkn 421 | 11 04 27.31 | +38 12 31.8 |
| 1156+295 | 4C 29.45 | 11 59 31.83 | +29 14 43.8 |
| 1226+023 | 3C 273 | 12 29 06.70 | +02 03 08.6 |
| 1229-021 | 4C -02.55 | 12 32 00.00 | -02 24 05.8 |
| 1510-089 | PKS 1510-08 | 15 12 50.53 | -09 05 59.8 |
| 1611+343 | DA 406 | 16 13 41.06 | +34 12 47.9 |
| 1633+382 | 4C 38.41 | 16 35 15.49 | +38 08 04.5 |
| 1641+399 | 3C 345 | 16 42 58.81 | +39 48 37.0 |
| 1739+522 | 4C 51.37 | 17 40 36.98 | +52 11 43.4 |
| 2230+114 | CTA 102 | 22 32 36.41 | +11 43 50.9 |
| 2251+158 | 3C 454.3 | 22 53 57.75 | +16 08 53.6 |
| 2254+074 | PKS 2254+074 | 22 57 17.30 | +07 43 12.3 |
| 2356+196 | PKS 2356+196 | 23 58 45.98 | +19 55 18.6 |
The quasar PKS 0420-01 was detected by EGRET, which revealed a variable
-ray emission (Lin et al. 1996).
All our data on this source in the R, V, and B bands are shown in Table 2; in Fig. 1 (click here) the last-season light curve in the R band is presented. The R magnitudes are calculated by a provisional calibration; we can thus confirm the historical maximum on September 15, 1995 (Villata et al.\ 1997a), when the source reached R=14.15. As for the V and B magnitudes, they are normalized to that of Star A of our reference stellar sequence.
Figure 1: Light curve of PKS 0420-01 in the R band
This object continues to show an intense activity: after the fall of 
in 40 days from the historical maximum, the brightness started to
increase again up to R=14.98 on December 11, 1995 (
), which means a magnitude variation 
in 48
days, with a steepest increase of 
in
6 days 
.
After a new drop, the source brightness
oscillated around 
.
The light curve in the R band of PKS 0422+00 is plotted in Fig. 2 (click here), showing the results of two observational seasons. The complete list of our data in the R, V, and B bands is given in Table 3. The choice of the photometric sequence for the source magnitude calibration is described in Massaro et al. (1996). Data from the first observational season have been already published in that paper as part of a national collaboration on this source. The new data confirm the pronounced oscillating behaviour of this source, variations in the B band being wider than those in the R band. Massaro et al. (1996) have already shown that the spectral index appears to be related with the source brightness, the spectrum becoming flatter when the flux increases. In order to check this point with the new data added, colour indexes versus R magnitude are shown in Fig. 3 (click here): one can see that B-R clearly decreases with increasing source brightness, even if this tendency is not so evident when looking at the V-R versus R plot.
Figure 2: Light curve of PKS 0422+00 in the R band
Figure 3: Colour indexes of PKS 0422+00 versus R magnitude
Because of its faintness, we observed PKS 0528+134 in the R band only; the corresponding last-season light curve is shown in Fig. 4 (click here), whereas the complete set of data is given in Table 4. The source magnitude is normalized to that of our reference Star A.
Figure 4: Light curve of PKS 0528+134 in the R band; the box indicates the EGRET
pointing period of October 17-31, 1995
Small amplitude oscillations characterize the source luminosity behaviour; only
at the end of the observational period we registered a brightness increase of
in 15 days.
We made only four observations of PKS 0805-07, in the period around the EGRET pointing of January 2-16, 1996. The data, in the R band, are given in Table 5. Magnitudes are rescaled to that of Star B in our reference star sequence.
The noticeable variability of OJ 248 has already been reported in
Villata et al. (1997a), where calibration of the photometric
sequence was also given; data from the new observational season show an even
more intense activity. As can be seen from the light curve in the R band
plotted in Fig. 5 (click here) and tabulated in Table 6, the source brightness
reached a maximum of R=15.60 on November 18, 1995. As far as we know, this
is the highest optical level ever registered for this source. A very steep
brightness decrease followed the peak: a variation of 
in
about 20 hours was observed as part of a wider decrease of 
in
16 days. Finally, another peak of R=16.25 was reached, with a brightness
rise of 
in 19 days and a decrease of 
in 15
days. Two data in the V band and three data in the B band are also given
in Table 6.
Figure 5: Light curve of OJ 248 in the R band
The complete set of our data on 0836+710 is given in Table 7. The last-season
light curve in the R band is shown in Fig. 6 (click here), where the box indicates the
period of EGRET pointing. Magnitude calibration has been performed according to
the photometric sequence published in Villata et al.
(1997a). Only small amplitude magnitude oscillations were reported
in that paper, the average brightness level being 
and the
maximum overall variation 
. Data from the new
observational season reveal a more pronounced variability, as well as the
historical brightness maximum (R=16.09) on November 20, 1995.
Figure 6: Light curve of 4C 71.07 in the R band; the box shows the EGRET pointing
period of April 3-23, 1996
The results of our observations are reported in Table 8, where magnitude calibration is based on the photometric sequence published in Villata et al. (1997a). Only four data in the R band were taken in the last observational season, showing no indication of significant variability.
No significant variability of the -ray flux from Mkn 421 was detected
during phases 1 and 2 of EGRET observations
(Sreekumar et al. 1996). This
source is known to be very active at TeV energies.
Gaidos et al. (1996) have
recently reported on observations by the Whipple telescope: two dramatic
outbursts were detected in May 1996, the flux reaching the highest level ever
recorded in the first flare.
All our data in the R, V, and B bands are given in Table 9. Figure 7 (click here) shows the light curve of Mkn 421 in the R band; magnitudes are normalized to that of Star B of our stellar reference sequence. We have four data around middle May (the last four ones), just after the first flare observed by Whipple on May 7, but at the time of the second flare, registered on May 15. We cannot confirm a relevant brightness increase during the second TeV flare, and we can say nothing on the previous period because of lack of data. We notice however that when the second TeV flare was detected, the optical state was relatively high (see also Weekes 1996).
Figure 7: Light curve of Mkn 421 in the R band; the box indicates the EGRET pointing
period
This quasar shows rapid variability in the -ray emission
(Sreekumar et al. 1996). Its light curve in the R band is
shown in Fig. 8 (click here), where boxes
indicate periods of EGRET pointing. The complete set of our data in the R,
V, and B bands is given in Table 10; source magnitude calibration was
discussed in Villata et al. (1997a).
Figure 8: Light curve of 4C 29.45 in the R band; boxes indicate EGRET pointing
periods
A great activity characterizes the source luminosity behaviour: when
considering the two observational seasons, the maximum magnitude variation is
and 
. By looking at the new data in
Fig. 8 (click here) one can see that in December 1995 a brightening of more than
a magnitude occurred in 8 days (
, 
); after
the peak of December 20, a brightness decrease of almost two magnitudes
occurred in 39 days (
, 
), within a total drop
of 
in 73 days. Data taken in the period corresponding to the
EGRET pointing of February 20 - March 5, 1996 are also plotted in
Fig. 9 (click here). At that time the source was in a low state and moderately
active; some colour change was registered too. In particular, a rapid
brightening of 
in 3 days was observed at the end of the
pointing period.
Figure 9: An enlargement of the 4C 29.45 light curve in the R band to show data in the
EGRET pointing period of February 20 - March 5, 1996
We notice that very different optical states correspond to the two EGRET
pointings shown in Fig. 8 (click here); this should set serious constraints on theoretical
models when optical data will be compared to the ones.
This quasar was detected in a high -ray state during EGRET phase 3
observations (von Montigny et al. 1996).
The results of our monitoring on 3C 273 in the R, V, and B bands are reported in Table 11; the light curve in the R band is plotted in Fig. 10 (click here). Periods of EGRET pointing are shown by boxes.
Figure 10: Light curve of 3C 273 in the R band; boxes indicate EGRET pointing periods
This source did not present relevant magnitude variations. We notice that one of the two reference stars that we chose for the source magnitude calculation (Stars E and G of the Smith et al. (1985) photometric sequence) is probably variable. This can be inferred by looking at the subset of Fig. 10 (click here): there is a decreasing trend in the deviation from the mean magnitude difference between Star E and Star G. By comparison with the other two stars in the Smith et al. (1985) sequence, we verified that the candidate to variable star is Star E.
We could collect only a few data on 4C-02.55; its light curve in the R band is plotted in Fig. 11 (click here), where boxes indicate EGRET pointing periods. Magnitude calibration was given in Villata et al. (1997a). All our data in the R, V, and B bands are presented in Table 12. No significant brightness variation was found.
Figure 11: Light curve of 4C-02.55 in the R band; boxes indicate EGRET pointing
periods
The results of our observations of PKS 1510-08 in the R, V, and B bands are reported in Table 13. The light curve in the R band is shown in Fig. 12 (click here), where magnitude calibration is performed according to Villata et al. (1997a).
Figure 12: Light curve of PKS 1510-08 in the R band
While in the first observational season this source did not show noticeable
variations, a brightness rise of 
in 13 days was detected at the
end of February 1996, with a final increase 
in 24 hours up to
the peak of R=15.02 on March 1 (
).
Table 14 collects our observations of 1611+343 in the R, V, and B bands; the light curve in the R band is plotted in Fig. 13 (click here), where the two boxes show periods of EGRET pointing. The source magnitude calibration was done according to Villata et al. (1997a). No long time scale trend was detected for this source, but a short time scale flickering.
Figure 13: Light curve of DA 406 in the R band; boxes show periods of EGRET pointing
All our observational data on 1633+382 in the R, V, and B bands are given in Table 15. Figure 14 (click here) shows the light curve in the R band; the two boxes indicate EGRET pointing periods. Magnitude calibration was performed by using the photometric sequence presented in Villata et al. (1997a).
Figure 14: Light curve of 4C 38.41 in the R band; boxes indicate EGRET pointing
periods
In the first season a big flare was detected in the R band, the source
reaching a historical maximum (Villata et al. 1997a;
see also Bosio et al. 1995; Raiteri et al. 1996)
with very steep brightness increase and decrease. A noticeable intranight
variability was observed too. No particular feature was noticed during the
last observational season probably because of lack of data; the range of
magnitude variation was 
.
The results of our optical monitoring of 3C 345 in the R, V, and B bands are reported in Table 16. Figure 15 (click here) shows the light curve in the R band; two boxes indicate EGRET pointing periods. Stars D and E from the Smith et al. (1985) photometric sequence have been used for the source magnitude calibration.
Figure 15: Light curve of 3C 345 in the R band; boxes show periods of EGRET pointing
The most interesting feature in the new data is the brightness increase 
in the last 54 days.
Our data in the R, V, and B bands are given in Table 17 and the light curve in the R band is shown in Fig. 16 (click here). The source magnitude is normalized to that of Star A in our stellar reference sequence. Only three data belong to the new observational season.
Figure 16: Light curve of 4C 51.37 in the R band
This quasar was detected as a strong -ray emitter by EGRET, with a
variable photon flux (Lin et al. 1996).
The complete list of our data on CTA 102 is presented in Table 18. The light curve in the R band from June 1995 to January 1996 is plotted in Fig. 17 (click here), where the EGRET pointing period is indicated by the box. The source magnitude is normalized to that of Star B of our stellar reference sequence.
This source did not present big magnitude variations. On November 19, 1995,
nine frames were taken in the R band, showing an oscillating behaviour with
.
Figure 17: Light curve of CTA 102 in the R band; the box indicates the EGRET pointing
period of November 28 - December 14, 1995
Blazar 3C 454.3 is another example of a strong and variable -ray source
(Lin et al. 1996).
The data of our monitoring of 3C 454.3 in the R, V, and B bands are presented in Table 19; the last-season light curve in the R band is shown in Fig. 18 (click here), where the box indicates the EGRET pointing period.
Figure 18: Light curve of 3C 454.3 in the R band; the box indicates the EGRET pointing
period of November 28 - December 14, 1995
An interesting feature is that on November 18, 1995 a peak of R = 15.63 was
detected, immediately followed by a brightness decrease of 
in
1.7 hours. In the same time, the source brightness in the V band varied by
only, which is inside the errors. Rapid variations in the R
(but not in the V) band were already reported by
Villata et al. (1997a).
The BL Lac PKS 2254+074 was observed in the R band only; the light curve is given in Table 20 and Fig. 19 (click here), where magnitudes are normalized to that of Star D of our reference sequence.
Figure 19: Light curve of PKS 2254+074 in the R band
The new data confirm the fast variability already noticed in
Villata et al. (1997a). In particular, a brightness decrease
of 
in 21 hours is visible on November 18-19, 1995.
The source PKS 2356+196 was observed in the R band only; data are presented in Table 21 and Fig. 20 (click here), where magnitudes are given with respect to that of Star A in our stellar sequence. The box in Fig. 20 (click here) indicates the period of EGRET pointing.
Figure 20: Light curve of PKS 2356+196 in the R band; the box indicates the EGRET
pointing period
No significant brightness variation was found.