In this section, all the observed sources are briefly introduced by quoting the
main observational information available in the optical (and 
) band and
the corresponding journals of observations are presented. Tables and plots of
the light curves are shown, where the source magnitude is given as the average
between those derived with respect to the two reference stars presenting the
smallest variations in their magnitude difference: the deviation from the mean
magnitude difference is shown in the figure subsets. In case that at least one
of the two objects has a brightness comparable with that of the source, these
deviations can be assumed as reliable error estimates. In practice, errors are
calculated as the maximum between the above deviation and a given value (from
0.02 to 
) corresponding to the typical uncertainty for the
considered source and filter. For intranight variability, the lower limit to
the error may be diminished.
In the following, finding charts with identification of the reference stars are also shown for those sources for which the calibration of a new, unpublished photometric sequence was performed. The relative star magnitudes in the R, V, and B bands are given in Table 4 (see footnote to the title).
Usually, the best sampled light curves are those in the R band and the worst ones are those in the V band. When the number of observations in the V band was lower than four, they have been collected in Table 5.
Figure 1: Light curve of S2 0109+224 in the R band; the box
indicates the EGRET
pointing period
Figure 2: Light curve of S2 0109+224 in the B band
The most impressive optical variation of this source was registered in 1943
when, after an outburst, its brightness decreased by 
in 1
year (Pica 1977). The object was monitored by
Pica et al. (1988) from 1976 to
1988. During the first 4 years it showed a B magnitude around 16; in
1980-1981 its luminosity decreased by 
and maintained this value
until 1987, except for one flare in fall 1981. A flickering of 0.6-
characterized the whole observational period. The total variability range
was 
, with minimum and maximum values of 15.48 and 
. In their optical monitoring program, Xie et al. (1994) found brightness
variations in the period from 1985 to 1991 between 16.78 and 
in the V band and from 17.36 to 
in the B band, with a
flare of 
in V and 
in B registered in 70
minutes on December 9, 1985. Other observations of 0109+224 were performed by
Sillanpää et al. (1991) from 1985 to 1989, showing a variability range of
about 
in the V band, and by Valtaoja et
al. (1993) from 1990
to 1992, who registered a V magnitude oscillating between 16.32 and 15.05.
Our data in the R, V, and B bands (see Tables 6-5 and 7, respectively,
and Figs. 1 (click here) and 2 (click here)) confirm the oscillating behaviour of this source, with a
maximum variation of 
in the R band, where we have no
calibration of the field comparison stars. In the B band, by using the
photometric sequence by Miller et al. (1983), the brightness varies between
16.52 and 
, showing values comparable with those of the previous
campaigns. The box in Fig. 1 (click here) shows the pointing period of the Energetic Gamma
Ray Experiment Telescope (EGRET) on board CGRO, during which the optical data
reveal a sensible variation, slightly wider in the B band (see Fig. 2 (click here)).
Another different behaviour between the two bands can be found in the last
data: on 
(
) the source was relatively fainter
in B than in the other two bands (B-V=0.74), as can also be deduced by
comparing the following brightening 
with 
and
, leading to B-V=0.47 on 
. Moreover, in the last
night we have the only sensible intranight variation: 
in 3.5
hours.
Figure 3: Light curve of PKS 0420-014 in the R band
Figure 4: Light curve of PKS 0420-014 in the B band
Figure 5: Intranight light curve of PKS 0420-014 in the R band
This quasar has revealed a strong variability in the optical
band. Webb et al.
(1988) present its light curve from April 1969 to January 1986: the source is
very active and exhibits variations up to 
on a few year time
scale. A noticeable flare was detected in late 1979, when a 
increase in 5 days was registered, followed by a 
decrease in 23
days. A strong variability also comes out from the 20 year light curve (from
1970 to 1990) reported by Smith et al. (1993). Their data, in the photographic
band, show a maximum magnitude variation of 2.8 and three maxima at the
beginning of 1975, 1978, and 1982. Other three maxima were observed at the
beginning of 1990, 1991, and 1992 by Wagner et al. (1995), who also noticed
fast flux variations with time scales of the order of 1-10 days. The flare of
February-March 1992 was the highest optical state observed until then (
in the R band); in that period EGRET registered the highest
flux density. Moreover, since low fluxes or non detections at 
energies correspond to low optical states, a direct correlation between the
optical and 
emissions was suggested.
We have no calibration of the field comparison stars of this source;
consequently, in Tables 8-10 and in Figs. 3 (click here)-5 (click here) the source magnitude is given
as the deviation from the minimum value registered during the monitoring
period. Our data confirm the strong variability; the most noticeable variation
was the fall of 
in 40 days observed from September 15 to
October 25, 1995 in the R band. A rough calibration leads to the estimate
R=14.2 for the peak registered on September 15, 1995 (
), which
would thus represent the highest optical state ever seen.
We also checked for microvariability in the R band on October 25-26, 1995
(
): the steepest variation detected was 
in 40
minutes (see Fig. 5 (click here)).
Figure 6: Light curve of OG 147 in the R band; boxes indicate EGRET
pointing periods
Figure 7: Enlargement of the solid box shown in Fig. 6 (click here),
including data simultaneous
with the EGRET pointing of October 17-31, 1995
The peculiarity of this quasar is that the density of the neutral hydrogen
column derived from X-ray observations is very high, so that its optical
emission must be strongly absorbed, probably by the host galaxy and/or by a
galaxy on the line of sight (Zhang et al. 1994). The source has been pointed
by EGRET several times, revealing a very intense, strongly variable,
-ray emission and a steep spectrum. This latter feature is confirmed by
the COMPTEL detection (Collmar et al. 1994).
Because of its optical faintness we took images of OG 147 in the R band only,
with exposure times of typically 
. No calibration of the reference
stars was available. The results are shown in Table 11 and in
Figs. 6 (click here) and 7 (click here),
where the source magnitude is normalized to its minimum value. During the
observational period, the maximum variation of the source brightness was 
, while the fastest one was an increase of 
in 1
day (
-1016). In Fig. 6 (click here) the dashed boxes indicate pointing periods
by EGRET in which we have no data because of the solar conjunction, while the
solid box includes data that we took around the EGRET pointing of October
17-31, 1995 (
-1022). These data are better shown in Fig. 7 (click here): in the
middle of the EGRET period we detected both the fastest variation and the
highest source brightness.
Figure 8: Light curve of OJ 248 in the R band
Figure 9: Finding chart of OJ 248
There is not much information on this source in the literature. Previous data in the V band give magnitudes of 17.26 (Hewitt & Burbidge 1987) and 17.5 (Maoz et al. 1993). Nothing was known about possible flux variations. Redshift estimates from the emission lines have produced different results: on one side Hewitt & Burbidge (1987) obtained z=0.939 [which is close to the value of 0.941 derived by Steidel & Sargent (1991)], while Véron-Cetty & Véron (1987) got z=2.05.
The results of our monitoring campaign in the R band are shown in Table 12
and Fig. 8 (click here), while two data in the B band are presented in Table 13 and the
only observation in the V band in Table 5. The photometric sequence that we
chose is shown in Fig. 9 (see footnote to the title) and the corresponding
stellar magnitudes are given in Table 4. The source has revealed a noticeable
variability, with a decrease of 
in the first 63 days and a
subsequent increase of 
in 58 days.
Figure 10: Light curve of 4C 71.07 in the R band
Figure 11: Finding chart of 4C 71.07
The interesting features of this quasar are the strong 
-ray emission
and fast optical variability of small amplitude. Its optical monitoring has
been started in 1989 by Wagner and coworkers (Wagner et al. 1990), who found
variations in the R band below 
, inside the errors. An optical flare
was detected in February 1992, with an increase 
and a
maximum brightness R = 16.54 (von Linde et al. 1993). After that, and up to
mid 1993, the source magnitude has remained more or less constant around R =
16.6 (Schramm et al. 1994). The source was observed by EGRET just before and
just after the flare, showing a doubling of the flux and a steep spectrum
(Thompson et al. 1993).
Tables 14 and 15 and Fig. 10 (click here) show our results from November 16, 1994 to April
14, 1995. One observation in the V band is reported in Table 5. The mean
magnitude in the R band was 
and no appreciable brightness
variations were detected. The source magnitude calibration was performed by
adopting the photometric sequence presented in Fig. 11 and in Table 4.
Figure 12: Light curve of 3C 216 in the R band
Figure 13: Finding chart of 3C 216
This was one of the first quasars to be discovered; its classification is controversial, since 0906+430 presents some typical features of BL Lacertae objects (high and variable optical polarization) as well as those of highly polarized quasars (HPQ) (strong emission lines). According to Fejes et al. (1992) the source must be considered an optically violent variable (OVV), i.e. an active galactic nucleus (AGN) with violent optical variations and high polarization, but monitoring studies have been lacking so far, hence it is not known whether the source is strongly variable or not.
The light curve of 0906+430 that we obtained (in the R band) is plotted in Fig. 12 (click here) and the corresponding data are given in Table 16. Fig. 13 shows the field of 3C 216; the estimated magnitudes of the four reference stars we have adopted are given in Table 4. One of them (Star B) has been already calibrated by Penston et al. (1971), who found V=14.02 and B=14.93. The other star calibrated by the above authors, Star A in Fig. 13, was not used for the analysis since it often saturates the pixel capacity in our R frames. During our observational period the source has always been faint, with a maximum magnitude variation of 0.80.
Figure 14: Light curve of Mkn 421 in the R band; the box indicates the
EGRET pointing
period
Figure 15: Light curve of Mkn 421 in the B band
Miller (1975) reports the light curve of Mkn 421 in the B band from 1899 to
1975. The main features are: the presence of a luminosity maximum on January
19, 1934, when the source reached B = 11.6; the high luminosity state shown
in 1901, 1916, and 1936, with B < 12.5; the fast brightness decline of 1.6
mag in 16 days observed in January 1942. The maximum magnitude variation over
the considered period was 
. According to
Xie et al. (1988), a
variability time scale of 2.5 hours can be inferred (on January 13, 1986 a
increase in the B band was registered in that time), from
which the authors estimate the mass of the Mkn 421 black hole to be about 
. More recently, another flare was announced by
Hurst (1992), who observed a 
increase in the V band in about
40 days. Mkn 421 was the first BL Lac object to be revealed in the 
band; its 
flux is however weak. It is also the only AGN,
besides Mkn 501, that was detected at TeV energies by the Whipple
Observatory High Resolution Atmospheric Cerenkov Camera
(Kerrick et al. 1995). In particular, a flare at TeV
energies was registered in May 1994; in the same period the ASCA
satellite detected a high X-ray flux, while EGRET did not observe a
flux change and the International Ultraviolet Explorer (IUE)
satellite measured a normal ultraviolet (UV) flux. Another
multifrequency campaign was performed in April-May 1995 when
observations by the Whipple and ASCA observatories showed a high state in both the energy bands (Takahashi et al.
1995; Buckley et al. 1995).
A photometric sequence was calibrated by Véron & Véron (1975) in the B band, but all their stars (except Star 1) are too far from the source to be included in our frames. Therefore, the data in Tables 17 and 18 and in Figs. 14 (click here) and 15 (click here) are expressed as magnitude differences with respect to the minimum value registered.
The maximum magnitude variation that we detected is about 0.4 in both the R
and B bands, the steepest variation being 
in 1 day (R
band). The box in Fig. 14 (click here) shows the EGRET pointing period.
Figure 16: Light curve of 4C 29.45 in the R band; the box indicates the
EGRET pointing period
Figure 17: Light curve of 4C 29.45 in the B band
This quasar has been recognized to be an OVV (Stockman 1978). After many years
of inactivity followed by a few years of slowly increasing activity, in spring
1981 a strong outburst was detected by Wills et al. (1983). The brightness
increase was 
and the high activity state lasted about 1 year;
in this period the optical light curve revealed a small amplitude variability
on half an hour time scale. Another big outburst of 
was
observed in 1985 (Webb et al. 1988). More recently, activity in the
-ray, optical, and millimeter bands has been reported
(Webb et al. 1995), the EGRET data revealing a flare in the 
-ray emission of several
(hence about one order of magnitude greater
than the previous detection in 1991 and one of the highest fluxes ever observed
in an AGN) on April 27, 1995.
The journals of our observations in the R and B bands are shown in Tables 19 and 20, respectively; the corresponding light curves are plotted in Figs. 16 (click here) and 17 (click here). One observation in the V band is included in Table 5.
For magnitude calibration we used the stars of the photometric sequence by Smith et al. (1985), but adopted our magnitude estimates (see Table 4), which are in good agreement with the values derived by the above authors.
The source has always been actively variable over all the monitoring period
(see also Raiteri et al. 1996), showing a maximum oscillation amplitude of
about 
in both the R and B bands; the steepest variation
was a decrease 
in 2 days (
-822). A not negligible
intranight variability was seen on April 7: a quasi linear decrease having a
slope comparable with that of the above steepest drop.
The box in Fig. 16 (click here) indicates the period of EGRET pointing: unfortunately we
have only a couple of data and in particular no data simultaneous with the
flare occurred on April 27, but one observation on April 28 reveals a
high optical state.
Figure 18: Light curve of 3C 273 in the R band
Figure 19: Light curve of 3C 273 in the B band; the box
indicates the EGRET pointing period
This is a very bright and consequently very famous quasar. Among its main
features one can quote the optical jet and the existence of a UV excess (blue
bump) in the spectrum, whose luminosity is comparable with or even greater than
the 
one. The source does not show a high optical activity: variations
less than 
were reported after several years of observations. An
optical flare was detected at the beginning of 1983 (Sadun
1985; Sillanpää
et al. 1988), the visual magnitude reaching 12.17. The first detection of 3C
273 at 
energies was obtained by the COS B satellite in July 1976 and
then in June 1978. Subsequently, it was observed by the instruments OSSE
(
-
), COMPTEL (1-
), and EGRET
(
-
) on board CGRO. This allowed to reconstruct the
spectrum of the quasar in the 
band (Johnson et al. 1995).
The results of our monitoring campaign for 3C 273 are presented in Tables 21-23 and in Figs. 18 (click here) and 19 (click here). For the magnitude calibration we have adopted the field comparison stars chosen by Smith et al. (1985). Our photometric calibration in the R and B bands (see Table 4) gave results in good agreement with those determined by the above authors. The box in Fig. 19 (click here) shows the period of EGRET pointing. We have not observed significant variations in the source brightness during all the considered period.
There is not much information about this quasar in the
literature. Pica et al.
(1988) report on 34 observations in the photographic system relative to the
period from May 1971 to April 1987: the average magnitude is 16.72 and the
minimum one is 16.28, the total observed range of variation being 
. Wilkes et al. (1994) quote B=17.23 and V=16.75 from the catalogue of
Véron-Cetty & Véron (1987).
Figure 20 shows the comparison stars we chose in the field of PKS
1229-021; their magnitudes are reported in Table 4. Our data in the R band are
given in Table 24; one observation in the V band is included in Table 5. As can be
seen, we found a mean magnitude in the R band of 16.42 with no significant
variations, and a value of 
in the V one, in agreement with
published data.
Figure 20: Finding chart of PKS 1229-021
Figure 21: Light curve of 3C 279 in the R band
Figure 22: Light curve of 3C 279 in the B band; the box
indicates the EGRET pointing period
This is a very active quasar, showing flares at all wavelengths. It has been
classified as an OVV by Webb et al. (1990), who present its historic light
curve from 1927 to 1990. In the period before 1951, a series of flares with
-4 are discernable, as well as a very violent outburst in 1938,
when the source brightness rose from 
to B=11.27 in about 1.5
years. After 1951 the object is less active, flares exhibiting a smaller
amplitude (
). A big outburst was observed in 1988, when the
magnitude B=12.13 was reached, with an increase from B=14.00 to the maximum
brightness in 24 hours. Other two flares were detected in 1989 and in 1992,
this latter being of particular interest since simultaneous observations in the
and UV bands were made (Netzer et al. 1994). In general, the 
emission of 3C 279 appears very variable, with a typical time scale of about 1
day. It has been detected by both EGRET and COMPTEL on board CGRO, allowing the
spectrum at 
energies to be derived.
The results of our observations are given in Tables 25-27 and in Figs. 21 (click here) and
22. Our data confirm the large variability of the source brightness: we
registered an overall 
variation in both the R and B bands.
The box in Fig. 22 (click here) shows the EGRET pointing period: a fall of 
was observed at that time.
The field reference stars that we chose for magnitude calibration are shown in
Fig. 23; their magnitudes are given in Table 4. We have to notice that Star B
is actually a variable object with a peculiar spectrum, presenting fast
variations especially in the B band (up to about 
in 1 day), and
thus it cannot be adopted as comparison star.
Figure 23: Finding chart of 3C 279
Figure 24: Light curve of PKS 1510-089 in the R band
Figure 25: Light curve of PKS 1510-089 in the B band
Figure 26: Finding chart of PKS 1510-089
Figure 27: Finding chart of DA 406
This quasar presents strong spectral analogies with 3C 273: in both sources a
pronounced UV excess, a very flat X spectrum, and a steep 
spectrum are
found. From the analysis of observational data from 1933 to 1952
Liller &
Liller (1975) report a maximum brightness variation of 
in the
B band. Between 1935 and 1945 the mean magnitude is 
; during 1946
the brightness increases and stabilizes around B=14.8 during 1947. In 1948
the quasar undertakes a period of intense activity, during which two maxima
separated by 57 days are observed: in one of these the source magnitude reaches
B=11.8. Later on the brightness decreases down to B=15 in 1952. Between
1968 and 1977 a further slow decrease is seen. After a maximum in 1987.3, the
quasar luminosity rapidly falls again (Pica et al. 1988).
The data we collected during our monitoring campaign are presented in Tables 5,
28, and 29, and in Figs. 24 (click here) and 25 (click here). The comparison stars adopted are shown in
Fig. 26; their magnitudes are given in Table 4. Small amplitude oscillations
are visible on short time scales, the total maximum variation being 
in both the R and B bands. A peculiarity of this source is the
evidence of a sensitively different trend of the R light curve with respect
to the B one.
Figure 28: Light curve of DA 406 in the R band
Figure 29: Light curve of DA 406 in the B band
Pica et al. (1988) have been observing this source
sporadically between 1980
and 1987, detecting a mean magnitude B=17.76 and a maximum variation 
. A more intense monitoring was performed by Smith
et al. (1993) from
1980 to 1991: their light curve can be described as the superposition of fast
flares on a slow growth. The minimum and maximum magnitudes registered are
V=16.8 and V=18.3. Tornikoski et al. (1994), in their study of possible
correlations between the radio and optical emissions, report optical fluxes
ranging from 0.27 to 
in the period 1983-1991.
The finding chart of this source is shown in Fig. 27, where field comparison stars are indicated; the calibrated magnitudes of two of them are given in Table 4. Our data in the R and B bands are shown in Tables 30 and 31 and in Figs. 28 (click here) and 29 (click here), while the only datum in the V band can be found in Table 5. As in the case of PKS 1510-089, wider luminosity variations are found on the shortest rather than on the longest time scales.
Figure 30: Light curve of 4C 38.41 in the R band
Figure 31: Intranight light curve of 4C 38.41 in the R band
Figure 32: Microvariability of 4C 38.41 in the R band
Figure 33: Finding chart of 4C 38.41
This quasar has also been classified as an OVV (Mattox et
al. 1993) because of
its strong optical variability. Barbieri et al.
(1977) report the source light
curve in the B band from May 1969 to May 1976; wide variations are visible on
both long and short time scales. In particular, a brightness decrease (from the
peak at B=15.85 observed on May 27, 1971) of 
in 4.02 years
was registered, while the maximum variations observed on shorter time scales
were the rise to and the drop from the same peak: 
in 11 and 20
days, respectively (see also Bozyan et al. 1990). As for its 
emission, 4C 38.41 is one of the most powerful sources detected by EGRET.
The violent optical variability of this source is confirmed by our data in
Tables 32 and 33 and in Fig. 30 (click here). As can be seen from the data in the R band,
after a period of quiet emission an outburst was detected with a brightness
increase of 
in 7 days, when, on June 27, 1995, the magnitude
reached 14.96. These are the maximum brightness (comparable with that of the
above quoted peak) and the steepest big variation ever observed for this quasar
(see also Bosio et al. 1995; Raiteri et al. 1996). The subsequent dimming
phase was extremely rapid too: a first decrease of 
was
registered in 21 hours, followed later by another one of 
in 2.9
days, with which the brightness returned to its ``normal" levels. Between these
two drops, we were able to detect also a noticeable intranight variability, as
is shown in Fig. 31 (click here) for 
, when an increase 
was
registered in 35 minutes. Other examples of microvariability are presented in
Fig. 32 (click here), where the data collected during 
and 
are plotted:
again one can see an increase of 
, this time in 53 minutes.
The reference stars that were adopted for the photometric calibration are reported in Fig. 33 and the magnitudes of three of them are listed in Table 4.
Figure 34: Light curve of 3C 345 in the R band
Figure 35: Light curve of 3C 345 in the B band
This quasar was recognized to be an OVV by Penston &
Cannon (1970). Bregman et
al. (1986) show optical data from 1965 to 1984 and notice two kinds of
variation: the first one is a flickering with oscillations up to 
on a few week time scale, the second one being represented by long term trends
such as the 
increase in the mean brightness between 1972 and
1982. Moreover, four outbursts are visible in 1967, 1969, 1971, and 1982 (see
also Schramm et al. 1993). In the period from 1971 to 1984 the variability
range is about 
in both the B and U bands, while in the V
band it is about 
. This discrepancy is explained by the authors
as possibly due to the effect of the 
bump on the B and U
bands and of the strong MgII line on the B one.
Observations reported by Schramm et al. (1993) show that after 1986 the
optical flux decreases, with no evidence of flares, until it reaches 
in May 1989. The above authors present data in the B, V, and R
bands taken at Calar Alto from 1988 to 1992. Until fall 1990, 3C 345 is in a
low optical state [an historical minimum is reached in May 1990, with B=18.66
(Kidger & Takalo 1990)]. In the period 1990-1992 three rapid bursts are
observed, with an overall brightness variation of 
. A
multiwavelength study of the 1991 outburst was performed by Webb et al.
(1994).
Microvariability was checked by Kidger & de Diego (1990) during one night in May 1989: ten frames were taken in 1 hour, revealing a variation of half a magnitude.
The results of our monitoring are presented in Tables 34 and 35 and in Figs.
34 (click here) and 35; one observation in the V band is included in Table 5. The source
magnitudes have been calculated with respect to Stars D and E in the sequence
given by Smith et al. (1985). Our light curves confirm the presence of small
amplitude oscillations on short time scales (1-3 weeks); in particular, an
increase of 
in 3 days was observed in the R band between
and 
, followed by a decrease of 
in 1 day
(
-912). The maximum magnitude variation over the whole monitoring
period was 0.67 in the R band and 0.49 in the less sampled B one.
Figure 36: Light curve of 4C 51.37 in the R band
Apart from the identification of the radio source 4C 51.37 with a quasar of
photographic magnitude 18.5 (Kühr 1977), the optical information about this
source is very poor in the literature [Cohen et al. (1977) give R=18.5 and
B=18.7]. Impey & Tapia (1990) found an optical polarization of 
.
This object was detected in the 
band by EGRET, with a maximum flux of
(von Montigny et al. 1995).
Without calibration of the reference stars, our data are presented in Tables 36
and 37 (click here) and in Fig. 36 (click here) as magnitude differences with respect to the minimum
value. The maximum variation in the monitoring period was 
. Some
intranight variation can be distinguished on 
and 
.
Figure 37: Light curve of CTA 102 in the R band; the dashed box
indicates the EGRET
pointing period
There are not many data in the literature about the optical behaviour of this
source. Pica et al. (1988) refer about 65 observations in the B band from
August 1973 to November 1987: the average magnitude is 17.66, the minimum value
17.26, and the variation range observed is 
; the most
significant flare was registered in 1978 and led to a 
brightness increase in 2 days. Wilkes et al. (1994) report B=17.75 and
V=17.33 from Véron-Cetty & Véron (1987).
We observed this object in the R band only; data are presented in Table 38
and Fig. 37 (click here) as differences with respect to the minimum magnitude observed. The
dashed box in Fig. 37 (click here) indicates the period of an EGRET pointing, when the
source was in the diurnal sky; another pointing was scheduled from November 28
to December 12, 1995, just after the monitoring period covered by the present
paper. The overall brightness variation was 
.
Figure: Light curve of 3C 454.3 in the R band; the dashed box
indicates the EGRET
pointing period, the solid one is enlarged in Fig. 39 (click here)
Figure: Enlargement of the solid box shown in Fig.
38 (click here); the period coincides with that
of the B light curve in Fig. 40 (click here)
Figure 40: Light curve of 3C 454.3 in the B band
Figure 41: Light curve of PKS 2254+074 in the R band
The light curve of 3C 454.3 in the B band from 1966 to 1979 is reported by
Lloyd (1984). The mean brightness is seen to
decrease from B=16.4 in 1966 to
B=17.25 in 1971. The source remains in a low optical state from 1971 to 1979,
when it suffers a 
flare. Data in the B band from 1971 to 1985
are presented by Webb et al. (1988); the above flare in fall 1979 is reported
as a variation of 
in 63 days. Subsequently, the flux slowly
decreases for about 3 years, showing small and fast variations similar to those
observed before 1979. A general flux increase is evident from 1983 onward.
Between November 1986 and January 1987, the average B magnitude of the source
is 16.56, with a variation of 
in 44 days
(Corso et al. 1988). A more recent study on the correlation between the radio and optical
emissions of PKS 2251+158 from 1980 to 1992 shows a big increase of the optical
flux at the end of 1988, followed by an increase of the radio one with a few
month delay (Tornikoski et al. 1994).
As for the 
emission, this is one of the few blazars to have been
detected by COMPTEL, together with PKS 0528+134, 3C 273, and 3C 279: all these
objects show a spectral maximum at MeV energies (Blom et al. 1995). Detections
with EGRET and OSSE were obtained too.
The results of our monitoring program are shown in Tables 39-41 (R, V, and
B bands, respectively) and in Figs. 38 (click here), 39 (click here) (R band), and 40 (B band). No
calibration of the reference stars has been performed so that magnitudes are
normalized to the minimum value. In the R light curve of Fig. 38 (click here) the dashed
box refers to an EGRET pointing; we have no data in that period because of the
solar conjunction. The solid box in the same figure indicates the best sampled
period (preceding the EGRET pointing of November 28 - December 12, 1995, out
of the monitoring period we present in this paper), which has been enlarged in
Fig. 39 (click here) and corresponds to that of the B light curve in Fig. 40 (click here). Some
behaviour difference can be seen between the two bands; moreover, the
noticeable intranight variation on 
is not confirmed by the V data.
Another fast variation of 
in 2.6 hours (
) can be found
only in the R curve, this time for the lack of data in the other two bands.
In any case the source has presented only small, short term variations, within
a total range less than 
.
This source shows a very variable flux in the optical band. In November 1990
Xie et al. (1994) observed a fall of 
in 41 minutes, when the
source reached the minimum brightness B = 18.38. However, a steepest
variation was registered by the same authors in November 1987: a decrease of
in 40 minutes.
A flare of 
was observed by Pica et al.
(1988) in 1981: the
peak magnitude was B=15.65, followed by a brightness decrease of 
in 18 days. A maximum variation of 
in 1.39 years is found
in their 43 point light curve in the B band from July 1979 to November 1987
(see also Bozyan et al. 1990).
Our data in the R band are shown in Table 42 and in Fig. 41 (click here). Since
calibration of the reference stars was not performed, the source magnitude is
given as deviation from the minimum value. No trends on long time scales are
recognizable, while some fast variations were detected. The maximum variation
in the observational period was 
, and it occurred in 3 days. On
November 3, 1995 intranight variations were also observed, the steepest one
being an increase of 
in 10 minutes.
On this source we carried out also UBVRI (Johnson-Cousins) photopolarimetry
using the equipment (see Scaltriti et al. 1989) attached to the 
reflector of Complejo Astronomico El Leoncito (Argentina). The photopolarimeter
allows to perform linear and circular polarization measurements in the UBVRI
bands simultaneously. The design of the polarimeter is such that the sky
background polarization is directly eliminated; this has been found especially
valuable in the observations of faint objects and when there is moonlight.
Moreover, photometric light curves were obtained in each band.
The observations were taken during eight nights in the period
July 23 - August
1, 1995. The UBVRI photometric light curves are shown in Fig. 42 (click here) (left
panels), where nightly means are plotted against Julian Date; the corresponding
data can be found in Tables 43-47. The maximum error is 
on
(B and I bands), when the object was found particularly faint;
the mean error in the nightly means is 
. The light curves show
noticeable night to night changes, up to more than 
in 1 day,
reflecting thus the fast variability already noticed in both our monitoring and
literature data. Also the spectrum appears to change, as is evident by
comparing the various light curves. In particular, this is visible in the drop
between 
and 
, more pronounced in the U and B bands (1.22
and 
, respectively) in comparison with the other ones
(0.49-
).
The results on linear polarimetry (polarization P and position angle P.A.)
are shown in Tables 43-47 and in Fig. 42 (click here) (centre and right panels), where
nightly means are plotted. Taking into account the observational errors, P
does not change appreciably in the whole run, except the bump in the V band
observations obtained on 
-928; the increase of P is not visible in
the other bands. Analogously, no significant trend can be seen in P.A.,
except for the U band, where P.A. shows an abrupt increase, again on
-928, when, on the contrary, a slight decrease can be distinguished
in the other bands.
The mean polarization values during the observing run are 
,
, 
, 
, and 
; previous values
range from 
to 
(Kinman 1976;
Kühr & Schmidt 1990; Wills et
al. 1992). Excluding the three values of about 
(U band), the
average value of position angle in the whole run and for UBVRI is
; previous findings are 
by
Kühr & Schmidt (1990)
and 
by Wills et al. (1992). No evident correlation exists between
the photometric and polarimetric data.
Figure 42: Photometric light curves (left), linear polarization (centre), and
polarization angle (right) of PKS 2254+074 in the UBVRI bands
Wills & Wills (1976) report V=18.0 for this source. No information about its
optical behaviour is available in the literature. EGRET detected a maximum
flux of 
(Thompson et al.
1993).
Our data in the R band are shown, without calibration, in Table 48 and in Fig. 43 (click here), where the box indicates the EGRET pointing period. No significant variation has been observed.
Figure 43: Light curve of PKS 2356+196 in the R band; the box
indicates the EGRET
pointing period