The AGN BL Lacertae lies within an elliptical galaxy (Miller et al. 1978), at a distance $z=0.0688 \pm 0.0002$ (Miller & Hawley 1977). Ejection and evolution of four highly-polarized superluminal radio components moving on curved trajectories have been observed by Denn et al. (2000) with the VLBA.

One puzzling feature is that, although BL Lacertae stands as the prototype of a whole class of objects in which emission lines are absent or extremely weak, on some occasions broad H $\alpha$ and H $\beta$ emission lines were found in its spectrum, raising the issue of its membership to the class named after it (Vermeulen et al. 1995). Corbett et al. (2000) analysed eight spectra taken over a period of 30 months, from June 1995 to December 1997, and found that the equivalent width of the H $\alpha$ line varies approximately inversely with the optical continuum flux: this suggests that the broad-line region is likely not photoionized by the beamed synchrotron radiation of the jet (which nevertheless cannot be ruled out), but by radiation coming from the hot accretion disc.

BL Lacertae has been observed for more than a century in the optical band, and it is well known for its intense variability on both long (months, years) and short (days or less) time scales. Since the early work by Racine (1970), microvariability was detected in optical observations of this source. Miller et al. (1989) observed a $0.12 ~ \rm mag$ variation in $1.5\rm ~h$ .

Carini et al. (1992) reported on 17 years of optical monitoring, in which the source exhibited "erratic'' behaviour, its V magnitude varying between 14.0 and 16.0. From their B-V versus V plot no well-defined correlation between brightness and colour appeared, but a "bluer when brighter'' trend is recognizable. The authors searched for microvariability, and found several episodes of variations as fast as $0.1 ~ \rm mag~h^{-1}$ , the most rapid rate of change observed being $0.19 ~ \rm mag~h^{-1}$ .

BVRI photometry of BL Lacertae in 1993-1995 was presented by Maesano et al. (1997), confirming the spectral flattening for a flux increase.

A big observing effort was undertaken during the summer 1997 broad optical outburst, announced by Noble et al. (1997). Subsequent circulars reported on high flux levels also in the $\gamma$ - and X-rays (Hartman et al. 1997; Grove & Johnson 1997; Madejski et al. 1997; Makino et al. 1997).

Webb et al. (1998) carried out BVRI observations during the outburst, and found that variations were simultaneous in all bands, but of higher amplitude at the higher frequencies, and that there was a marginal evidence of a spectral flattening when the source brightens. These two latter features were recognized also in the microvariability events detected by Nesci et al. (1998) and Speziali & Natali (1998), presenting variations up to $0.4\rm ~mag~h^{-1}$ . VR observations on 11 nights in July 1997 were performed by Clements & Carini (2001), who detected nightly variations from 0.1 to $0.6 ~ \rm mag$ . They also found that BL Lac became bluer when brighter and commented that it is not clear whether the colour changes can be ascribed to the AGN or are rather due to a greater contribution from the underlying galaxy when the AGN is fainter. An extremely fast brightening of $0.6 ~ \rm mag$ in $40\rm ~min$ was detected by Matsumoto et al. (1999) on August 2, 1997, inside a larger flux increase of more than $1\rm ~mag$ between 17 and 19 UT, confirmed by Ghosh et al. (2000). In the same night, the decreasing phase of the flare was observed by Massaro et al. (1998) and Speziali & Natali (1998) as a variation of more than half a mag in about $2\rm ~h$ (see also Ghosh et al. 2000). Rapid and large-amplitude flux variations were also observed by Sobrito et al. (1999) and Tosti et al. (1999). These papers also contain extended light curves during the 1997 outburst.

During the optical outburst, the EGRET instrument on the Compton Gamma Ray Observatory revealed a $\gamma$ -ray flux more than 4 times the previous detection, and a harder spectrum than before (Bloom et al. 1997). Moreover, a noticeable $\gamma$ -ray flux increase observed on July 18-19 apparently preceded by several hours a brief optical flare. The RXTE and ASCA satellites detected an X-ray flux respectively 2-3.5 times and more than 3 times higher than measured by ASCA in 1995 (Madejski et al. 1999; Tanihata et al. 2000). The multiwavelength spectrum of BL Lac during the July 1997 outburst was examined by Böttcher & Bloom (2000) in terms of a homogeneous jet model.

Intraday variability was found also the next year, in summer 1998, when the source was in a fainter state (Massaro et al. 1999; Nikolashvili et al. 1999).

VRI photometry of BL Lacertae in 1997-1999 was presented by Fan et al. (2001): they detected microvariations with amplitude decreasing with increasing wavelength. They also analysed the correlation among bands, finding no time delay longer than $0.2\rm ~d$ .

An analysis of colour variability of BL Lac during the 1997 and 1999 outbursts was performed by Hagen-Thorn et al. (2002). They showed that in both cases the spectral energy distribution remained unchanged during the outburst, and that the spectrum was flatter in the more powerful outburst of 1997.

Since BL Lacertae is one of the few blazars for which time-extensive light curves exist, a number of investigations have been devoted to the search for periodicities in its flux variations.

Recurrent variations every 0.31, 0.60, and $0.88\rm ~yr$ were recognized by Webb et al. (1988) by means of a discrete Fourier transform (DFT) analysis of their light curves, extending from June 1971 to January 1985. No evidence of periodicity was found instead by Carini et al. (1992).

"Whitening'' of time series was the method used by Marchenko et al. (1996; see also Hagen-Thorn et al. 1997) to search for periodicities in a 20-year long light curve of BL Lac: they found that only a long-term component of $P = 7.8 \pm 0.2\rm ~yr$ is statistically significant.

**Table 1:** List of participating observatories by longitude.
Observatory	Tel. size (cm)	$N_{\rm obs}$	N_U	N_B	N_V	N_R	N_I
Kyoto, Japan	25	607	0	0	0	77	0
Osaka Kyoiku, Japan	51	1209	0	0	56	463	53
Mt. Maidanak (AZT-22), Uzbekistan	150	447	59	100	59	131	57
Mt. Maidanak (T60-K), Uzbekistan	60	743	99	128	133	0	0
Abastumani, Georgia (FSU)	70	2743	0	0	0	1253	0
Crimean, Ukraine	20	2470	0	38	67	159	78
Nyrölä, Finland	40	40	0	0	0	40	0
Skinakas, Crete	130	630	0	313	0	314	0
Catania, Italy	91	1071	132	132	132	0	0
Vallinfreda, Italy	50	92	0	24	5	34	26
Monte Porzio, Italy	70	112	0	25	30	28	29
Perugia, Italy	40	673	0	0	76	527	63
Torino, Italy	105	900	0	83	57	578	54
Guadarrama, Spain	20	18	0	0	0	9	0
Roque de los Muchachos (KVA), La Palma	60	2018	0	148	1	401	0
Roque de los Muchachos (NOT), La Palma	256	74	0	7	0	58	9
Bell Farm, Kentucky	60	1	0	0	0	1	0
St. Louis, Missouri	35	176	0	69	17	72	17
Sommers-Bauch, Colorado	60	8	0	0	3	3	0
Lowell, Arizona	180	323	0	119	8	185	8
San Pedro Martir, Mexico	150	25	0	6	9	7	3
Palomar, California	150	108	0	54	0	54	0
Clarke and Coyote, California	28	248	0	0	0	91	0
University of Victoria, Canada	50	889	0	0	0	363	0
Total		15 625	290	1246	653	4848	397

$N_{\rm obs}$ is the total number of observations done, i.e. the number of unbinned data; N_U, N_B, N_V, N_R, and N_I are the numbers of data points in UBVRI remained after discarding and binning some of the original data

In their study of the long-term optical base-level fluctuations in AGNs, Smith & Nair (1995) identified a best-fit, well-defined cycle of $7.2\rm ~yr$ for the baseline meanderings shown by the 20-year BL Lac light curve of the Rosemary Hill Observatory. It is interesting to notice that the mere application of the Fourier analysis led the authors to derive a period of $7.7\rm ~yr$ , in fair agreement with the results obtained by Marchenko et al. (1996).

Fan et al. (1998) analysed the historical optical light curve of BL Lacertae with the Jurkevich method and derived a long-term period of $\sim$ $14\rm ~yr$ .

2 BL Lacertae