3 Results

The results of our observations are presented in Tables 2-9 and Figs. 1-7, these latter showing light curves in the R band, since it is the best sampled one. The BeppoSAX pointings are indicated.

Magnitude calibration was obtained by adopting our own photometry (Villata et al. 1998a) for all sources but MS 0158.5+0019 and MS 0317.0+1834, for which we used the photometric sequences published by Smith et al. (1991). The source magnitude is calculated with respect to two reference stars in the same field; in the subsets to the figures we plot the magnitude difference between these two stars normalized to their mean magnitude difference. This gives information on the reliability of the reference stars in each frame. Most of the eight BL Lacs examined here are very faint, so that the magnitude uncertainties are often of the order of $0.1 \, \rm mag$.

3.1 MS 0158.5+0019

BL Lac MS 0158.5+0019 is a very faint source. Previous estimates of its V magnitude are: 17.96 (Stocke et al. 1991), 18.30 and 18.60 (Jannuzi et al. 1993), and 18.39 (Xie et al. 1996). Recently, in a study on the intraday variability in X-ray selected BL Lacs, Heidt & Wagner (1998) measured an average brightness R=18.5. They found no evidence of variability for this source during fifteen observations.

Our results are shown in Table 2. We have only one datum in the V band, indicating a magnitude of 18.60 on October 22, 1996.

The BeppoSAX pointing occurred on August 16-17, 1996.

3.2 MS 0317.0+1834

Stocke et al. (1991) reported V=18.12 for this source, and V=18.13 was detected by Jannuzi et al. (1993). Xie et al. (1992) found V=18.01 - 18.54 and recently V=17.82 - 18.28 (Xie et al. 1996); in this last work they noticed variability on short time scales. Variability was also found by Heidt & Wagner (1998), who give an average R magnitude of 17.7.

We have collected 11 data in the R band and 4 data in the V band from October 19, 1996 to February 17, 1997 (see Table 3 and Fig. 1). Our observations give R= 17.59 - 18.28 and V=17.97 - 18.72. A brightness increase of $0.51 \, \rm mag$ in 5 days was detected on February 5-10, 1997.

The BeppoSAX satellite pointed this source on January 15, 1997, within our observational period.

  

Figure 1: Light curve in the R band of MS 0317.0+1834

3.3 1ES 0347-121

As far as we know, the only optical photometric information on this source can be found in Schachter et al. (1993), who give a UK Schmidt $B_{\rm J}$ magnitude of 19.1.

Ten data in the R band (R=17.29 - 18.04) and two data in the V band (V=18.10 - 18.39) were taken between November 9, 1996 and February 10, 1997 (see Table 4 and Fig. 2). A brightness decrease of $0.75 \, \rm mag$ in a week is visible in the R light curve after the BeppoSAX pointing of January 10, 1997.

  

Figure 2: Light curve in the R band of 1ES 0347-121

3.4 [HB89] 0414+009

Observations of this BL Lac were done by Pica et al. (1988) in the B band from September 1984 to January 1987: they found B=17.16 - 18.09. McHardy et al. (1992) give R=16.64 and V=17.21; Hewitt & Burbidge (1993) report V=17.59 and B-V=0.48.

The BeppoSAX satellite pointed this source on September 21-22, 1996; our data cover the period October 19, 1996 - February 10, 1997 (see Table 5). The R light curve is shown in Fig. 3. With respect to the previous observations our data reveal a higher optical state: R=16.18 - 16.40, V=16.65 - 16.86, B-V=0.45 - 0.47.

  

Figure 3: Light curve in the R band of [HB89] 0414+009

3.5 1ES 0502+675

Only two sources of optical photometric data for this BL Lac can be found in the literature: a POSS O magnitude of 18.5 ($V \sim 18.0$) is found in Schachter et al. (1993), while Perlman et al. (1996) report V=17.0.

The BeppoSAX pointing occurred on October 6-7, 1996; we started to observe this source on October 18, 1996 (see Table 6). An increasing trend appears from the light curve in the R band shown in Fig. 4, with a maximum variation $\Delta R=0.58$.

  

Figure 4: Light curve in the R band of 1ES 0502+675

3.6 MS 0737.9+7441

Stocke et al. (1991) measured V=16.89 for this source, while Heidt & Wagner (1998) found an average R magnitude of 18.0 over 24 observations and no evidence of variability.

We collected 21 frames in the R band, 2 in the V band, and 1 in the B band between November 7, 1996 and April 5, 1997 (see Table 7); the BeppoSAX pointing was on October 29-30, 1996. The light curve in the R band is characterized by rapid flickering (see Fig. 5); in particular, an increase of $0.50 \, \rm mag$ in 3 days was detected at the end of the observational period.

  

Figure 5: Light curve in the R band of MS 0737.9+7441

3.7 Mkn 180

This is the most luminous object among the BL Lacs we are considering: Mufson & Hutter (1981) and Hutter & Mufson (1981) measured V=15.03 and B-V=0.76, while Hewitt & Burbidge (1993) report V=14.49 and B-V=0.67.

Our observations began on December 18, 1996, a week after the BeppoSAX pointing of December 10-11, 1996, and ended on May 8, 1997 (see Table 8 and Fig. 6). The source did not show a noticeable variability, but a weak flickering around a level lower than the previously reported ones (R=14.84 - 14.99, V=15.27 - 15.38, B-V= 0.63 - 0.74).

  

Figure 6: Light curve in the R band of Mkn 180

3.8 1ES 1517+656

This BL Lac is another object seldom observed in the optical band: the only photometric information is given by Perlman et al. (1996), who quote a V magnitude of 15.91.

We observed this source from February 18 to May 29, 1997, and have data simultaneous with the BeppoSAX pointing of March 5, 1997 (see Table 9). As can be seen from the light curve in the R band shown in Fig. 7, no relevant variability was detected. This source shows the flattest spectrum in the present sample: $V-R \sim B-V \sim 0.35$.

  

Figure 7: Light curve in the R band of 1ES 1517+656