The observations were carried out with a 0.5 m reflector telescope of
the Astronomical Station of Vallinfreda (Rome) at 850 m a.s.l. (since July
1995, formerly
at Oriolo) equipped with a CCD camera and the standard filters B,
V (Johnson) and R, I (Cousins). Exposure times were
generally 600 s in B and 300 s in the other bands.
The scale on our detector is 2.29 arcsec/pixel and we used
typically for our aperture photometry a radius of 3 pixels (12 Kpc at z=0.07),
so the host galaxy of BL Lac is substantially within our aperture.
The apparent magnitude
of this galaxy has been estimated 
, 
(Brown et al. 1989), one magnitude fainter than the lowest value recorded
by us in this period (V=15.9). Data reduction was performed
with the APPHOT tasks within the IRAF
package, using the comparison stars
given by Smith et al. (1985), which were all within our instrument
field of view. The (B-V) color indexes of these stars range form 0.8
to 1.3 and are therefore similar to that of BL Lac itself (
)
The observed magnitudes in the four photometric bands are given in Table 1 (click here).
No reddening corrections, which for BL Lac are relevant, were applied.
Repeated observations in the same night with the same filter were averaged
if their difference was less than two standard deviations. Only one case
of intranight variability is present in our sample (JD 2449924)
with an amplitude of about 0.1 mag. The number
of averaged observations is also listed in Table 1 (click here).
In the time interval monitored by our observations a significant variability is apparent in all the four bands; since the observations were not always performed in all the bands we cannot compare the variability ranges over the entire time interval. Considering only the epochs in which all the bands were observed we found that the variability range increases from 1.08 mag in I to 1.35 mag in B, confirming that the flux is more variable at higher frequencies, as observed in many other sources of this class.
In the second half of 1993 BL Lac was in a quite low state: our V data of the month of August interpolate very well those reported by Tornikoski et al. (1994) and confirm the good correlation between the optical and radio light-curve, which indicated a quiescent behaviour at 22 and 37 GHz.
In September and October 1994 the source was in a brighter state and likely had an outburst of a few days duration: in particular, on 1994 Sep. 7 (JD 2449603) the magnitudes B=14.85 and R=13.26 were reached, with an increase of 0.79 mag in B and 0.55 mag in I in three days; about one month after it was found at a level comparable to the initial one. Unfortunately, we were not able to perform a number of observations sufficient to achieve a detailed description of this episode.
Table 1: Observed magnitudes, no reddening correction applied
In 1995/96 BL Lac was found in a prolonged low state: the
brightest level observed (October 16) was only 
. We
observed the lowest flux (
) at the end of June; our
figure is only 0.15 mag fainter than that reported by Jeffries et al. (1995)
for 1995 June 4. In that occasion, spectroscopic observations (Vermeulen et
al. 1995)
showed in the spectrum of this object several emission lines and absorption
features, the latter originated by the associated galaxy.
Looking at the light curve we can also find
two minor flares around JD 2449933 and 2450007.
None of these flares is sufficiently well sampled to derive reliably the
rising and falling times in the different bands, and only in the
latter case we
could observe BL Lac also some days before and after the flare (
) so we can make an estimate of its amplitude (
).
The correlation coefficient between R and (V-I) is
0.54, and a smaller (V-I) corresponds to a brighter state. Actually
both flares are characterized by a significantly flatter spectral slope
(V-I=1.47) while the steepest (V-I) values (1.75) corresponds to
local minima in the light curve.
Unfortunately we have no 
data for the peak of the first (and
brighter) maximum.
In another occasion an interesting change of the spectral shape was observed between JD 2449957 and 2449958 (1995 Aug. 27-28) around one of the local minima. While the source brightened in V, R and I by, respectively, 0.34, 0.31 and 0.18 mag, it remained constant in B, at variance with the general behaviour of a wider flux variability at shorter wavelengths. This is reminiscent of the peculiar behaviour shown by OJ 287 during the minimum of 1989 (Kidger et al. 1991), when the variation amplitude was not a monotonic function of the wavelength.
