3 Observational results

3.1 BL Lac

On 12 October 1993, the source exhibited no convincing microvariability during the observation of one hour and 40 min. On the next night, 13 October, the source was about 0.1 mag brighter (see Table 1). On 22 October 1995 BL Lac was stable over the interval of our observation of about 2.5 hours. Six days later, 28 October, the source brightened by $\bigtriangleup V\sim 0.28$ mag. On 29 October 1995, the source was almost at the same brightness level as on the night before during our observation (see Table 1). Two month later, on 22 December 1995, the source became $\bigtriangleup V\sim 0.17$ mag brighter. On the next night, 23 December 1996, the source was a little fainter than on the night before (see Table 1). In August 1996, BL Lac undertook a small flare. From 14 to 24 August, the source brightened by $\sim\! 0.3$ mag and on 25 August it faded back by $\sim \!0.15$ mag (see Table 1). On 14 August we also observed a microvariation in the source. From JD = 2450310.1979 to 2450310.2153, the source brighten by $\bigtriangleup I=0.05$ mag in 20 min, then by $\bigtriangleup R=0.15$ mag in $\sim\! 55$ min (to 2450310.2635). On JD = 2450310.2635 it reached a maximum and then began to fade. The amplitude of this microvariability was $\sim\! 0.20$ mag (see Fig. 1 and Table 1). In early September 1996, the source was at the same brightness level as in August 1996, and the amplitude of variability was about 0.1 mag. During the observation of one hour and 40 min on 02 September, the source exhibited no evidence of microvariability (see Table 1). On 19 October 1996, the source was as bright as 14 September 1995, and showed no microvariability. On the next night, 20 October, the source was slightly fainter than on the night before (see Table 1). Over the interval of our observation from 7 to 11 November, the source gradually faded by $\sim\! 0.25$ mag (see Table 1). On 12 December 1996, BL Lac was $\sim\! 0.43$ mag brighter than on 11 November and gradually faded by $\bigtriangleup V \sim 0.23$ mag over the interval of the observation from 12 to 14 December 1996 (see Table 1).

  

Figure 1: Light curves of BL Lac on 14 August 1996. The open circles ($\circ$) are for I band data and filled circles ($\bullet$) for R

  

Figure 2: Light curves of BL Lac on 07 September 1997. The stars ($\ast$) are for V band data, filled circles ($\bullet$) for R

Combining the historical light curves constructed by Shen & Usher (1970) and the long term monitoring results by Webb et al. (1988), we can see that the range of variation for BL Lac in the B band was 12.4 to 17.2 mag. The faintest magnitude ever recorded is $B=17.99 \,\pm\, 0.11$ mag, $V=16.73 \pm 0.08$ mag (Carini et al. 1992); nevertheless, during about 17 years (1973 to 1990) BL Lac reached this faintest state only once (on 1980.367), the usual fainter states for BL Lac were those states around V=15.7 mag (see Fig. 13 of Carini et al. 1992). It is obvious that over the interval of 1995 and 1996 observing runs BL Lac kept at faint states, and was relatively stable. The amplitudes of intranight variation recorded in our observation were less than 0.3 mag (see Table 1). Better sampling observations for this source in 1995 by Maesano et al. (1997) also recorded no flares or fluctuation greater than half magnitude from the mean level.

During 1997 observing run, BL Lac became much brighter than it was in 1995 and 1996. On 7 September 1997, it was averagely 12.78 mag in the R band, about 1.9 mag brighter than it was on 14 December 1996 (see Table 1). During the observation on this night the source showed a steadily brightness increase of $\sim\! 0.55$ mag (see Fig. 2 and Table 1), which was probably a part of an intranight variation. On 3 October 1997, the source became about 1.0 mag fainter in the R band. No significant variability was observed during the observation of about one hour on this night (see Table 1). The observations on 4 December recorded a microvariation. From the beginning of our observation to JD = 2450725.0053 the source brightened by $\bigtriangleup I=0.11$ mag, then by $\bigtriangleup B=0.13$ mag, and then faded back by about the same amount till JD = 2450725.0565. The amplitude of this microvariability was $\sim\! 0.24$ mag (see Table 1). From 7 September to 4 December, it decreased $\bigtriangleup R \sim 1.4$ mag in its brightness. Nesci et al. (1998) monitored this source intensively in July 1997 for 9 nights (from 14 to 29 July) and found the source was never stable, but always showed a variability of at least 0.04 mag/hour, often with superimposed fast fluctuation on time scales of about an hour and with amplitudes of about 0.1 mag. The largest intraday variation had an amplitude of 0.7 mag. During their observation the source was at a bright state of about 14.0 mag in the V band. It was obvious that the source was at the stage of a large outburst during late 1997, accompanied with much more rapid flares and fluctuations than in 1995 to 1996 when the source was at faint states.

3.2 OJ 287

On 01 March 1994, the source was averagely 0.8 magnitude brighter than the mean level of B=16.5 mag (Kidger et al. 1995). From the beginning of our observation to JD = 2449413.1153 the source brightened by $\bigtriangleup B \sim 0.31$ mag, then faded back by $\bigtriangleup B \sim 0.14$ mag and $\bigtriangleup V \sim 0.06$ mag (see Fig. 3 and Table 1). On 03 March 1994, the source showed a decline of $\bigtriangleup B \sim 0.19$ mag from the beginning of our observation to JD = 2449415.0844, then recovered about the same amount in the B band to JD = 2449415.1384, and then began another decline till the end of our observation. (see Fig. 4 and Table 1).

  

Figure 3: The B and V band light curves of OJ 287 on 01 March 1994. The stars ($\ast$) are for V band data and triangles ($\triangle$) for B

On the average, the source was brighter on 03 March than on 01 march, and combining Figs. 3 and 4, we can see the variations on 1 and 3 March were probably parts of an intranight variation. On 04 December 1994, the source exhibited a microvariation of at least 0.23 mag. From the beginning of our observation (JD = 2449691.3398) to JD = 2449691.3757, the source first faded by $\bigtriangleup I=0.05$ mag, then by $\bigtriangleup V=0.06$ mag, and then by $\bigtriangleup B=0.12$ mag. On JD = 2449691.3757 it began to recover till the end of our observation (see Table 1). On 05 December, the source was a little brighter than on the night before. On 06 December the source was stable during the observation, but $\sim\! 0.2$ mag brighter than on the night before. Forty-eight days later, on 23 January 1995, the source became averagely 0.5 mag fainter in the V band and exhibited a steady decline of $\bigtriangleup R=0.17$ mag. On the next night the source showed a slow brightness increase of $\bigtriangleup R \sim 0.14$ mag which was superimposed by a microvariation of $\bigtriangleup R \sim 0.05$ mag (see Table 1). On 25 January 1995 the source brightened by $\bigtriangleup V \sim 0.13$ mag from the beginning of our observation to JD = 2449743.2431, then slowly faded by $\bigtriangleup V \sim 0.15$ mag to the end of our observation. The observation on 26 January 1995 recorded a local brightness minimum of V=15.34 mag which was brighter than the local maximum of V=15.41 mag on the night before, implying that the source had undertaken a brightness oscillation over the interval between 25 and 26 January (see Table 1). On 27 January 1995 the source was a little fainter than on 26, but showed no convincing variation during our observation. On 28 January 1995 the source exhibited a slow darkening of $\bigtriangleup R=0.12$ mag (see Table 1). The source was next observed on 22 March 1995 and $\sim\! 0.5$ mag fainter than on 28 January 1995. It exhibited a brightness oscillation with an amplitude of $\bigtriangleup R \sim 0.14$ mag (see Fig. 5 and Table 1). On the next night, 23 March, the source first faded by $\bigtriangleup R \sim 0.09$ mag and reached a local brightness minimum on JD = 2449800.125, then recovered 0.06 mag to JD = 2449800.1359, and then kept stable to the end of our observation (see Table 1). On 24 March 1995, the source was averagely 0.25 mag brighter than on the night before. The light curve on this night seems to show a brightness oscillation, but the amplitude is less than 3$\sigma$ (see Table 1). Over the interval from December 1994 to March 1995 the source gradually faded by about one magnitude. This was consistent with the observation by Sillanp$\ddot{\rm a}\ddot{\rm a}$ et al. (1996a) who recorded an outburst in this source which peaked around 10 November 1994 and ended in April 1995.

  

Figure 4: The B and V band light curves of OJ 287 on 03 March 1994. The stars ($\ast$) are for V band data and triangles ($\triangle$) for B

  

Figure 5: The R band light curve of OJ 287 on 22 March 1995

On 24 January 1996 the source was about one magnitude brighter than it was in March 1995 (see Table 1). On 15 February 1996, the source was stable during our observation and $\sim\! 0.09$ mag fainter than on 24 January 1996 (see Table 1). On 15 April 1996, the source was about 0.27 mag brighter than on 15 February 1996. On the next night, 16 April, the source became a little fainter and kept stable during our observation (see Table 1). From 16 to 18 April 1996, the source gradually brightened by $\sim\! 0.25$ mag, but exhibited no convincing variation on each night. The source was next observed on 02 January 1997 and became $\sim\! 0.6$ mag fainter. Nearly one year later, on 04 December 1997, the source faded by another 0.2 mag. During the observation on this night it also exhibited a microvariation of $\sim \!0.1$ mag (see Table 1). A month later, on 01 and 02 January 1998, the source became even fainter. On 01 January 1998, the source first faded by $\bigtriangleup B=0.06$ mag, then brightened back by $\bigtriangleup B=0.05$ mag, $\bigtriangleup V=0.06$ mag and $\bigtriangleup R=0.05$ mag to a local brightness maximum, and then began to fade to the end of our observation (see Table 1). The amplitude of this small flicker was at least 0.16 mag. During the observation of one hour and 45 min on 02 January 1998, the source showed no convincing variation (see Table 1). On 28 April 1998, the source was at the same brightness level as in January 1998 and showed no evidence of microvariability (see Table 1). Over the interval from January 1996 to April 1998 the source faded gradually by about one magnitude, back to the brightness level of March 1995. This is consistent with the observation by Sillanp$\ddot{\rm a}\ddot{\rm a}$ et al. (1996b) who detected a secondary outburst in this source which peaked just at Christmas 1995.

  

Figure 6: The R band light curve of PKS 0735+178 on 23 January 1995

3.3 PKS 0735+178

The source was rather stable during 04-06 December 1994. Only a slightly brightness decrease of $\bigtriangleup V\sim 0.08$ mag was observed between 04 and 05 December (see Table 1). Forty-eight days later, 23 January 1995, the source brightened by $\bigtriangleup V\sim 0.65$ mag, and exhibited a small flicker of $\bigtriangleup R \sim 0.13$ mag from JD = 2449741.1222 to 2449741.2115, and a smaller flicker of 0.06 mag from JD = 2449741.2115 to 2449741.2343 (see Fig. 6 and Table 1). On the next night the source was $\bigtriangleup V\sim 0.4$ mag fainter than the night before, but showed no convincing microvariation. Then two days later, 26 January, it brightened back by $\bigtriangleup V \sim 0.15$ mag. From 26 to 29 January the source was stable, no significant intranight variation and microvariation being detected (see Table 1). The source was next observed a year later, on 24 January 1996, and was as bright as on 29 January 1995. No convincing microvariation was detected on this night (see Table 1). Twenty-two days later, 15 February 1996, the source faded by $\bigtriangleup V\sim 0.2$ mag. On 13 December 1996, the source became about one magnitude fainter in the V band. On 02 and 03 January 1997, it was a little fainter than in December 1996. On 04 December 1997, the source reached the minimum of our monitoring, with $B\sim 17.64$ mag, near the faintest magnitude ever recorded of B=17.72 mag. During the observation of two hours and twenty minutes, the source showed no evidence of microvariability (see Table 1). Twenty days later, 24 December, the source brightened back by $\bigtriangleup V\sim 0.5$ mag. The observation on this night lasted over 4 hours, but detected no convincing microvariations (see Fig. 7 and Table 1). From 01 January to 24 February 1998 the source slowly faded by $\bigtriangleup V\sim 0.1$ mag. On 01 January 1998 the source was $\bigtriangleup V\sim 0.4$ mag brighter than on 24 December 1997 and exhibited a decline of $\sim\! 0.16$ mag from the beginning of the observation to JD = 2450815.2295. On 02 January during the observation of three hours and twenty minutes the source showed no convincing microvariation. On 22 February the source exhibited no evidence of microvariation during the observation of two and half hours. On 24 February, from the beginning of our observation to JD = 2450869.2220 the source was stable, but during last two hours of our observation the source showed a microvariation of $\sim\! 0.3$ mag (see Table 1).

  

Figure 7: Light curves of PKS 0735+178 on 24 Dec. 1997. The open circles ($\circ$) are for I band data, filled circles ($\bullet$) for R, stars ($\ast$) for V and triangles ($\triangle$) for B