1 Introduction

BL Lacertae objects are extremely active galactic nuclei (AGNs) showing rapid and large variations, high and variable polarization, having no or weak (EW<5 Å) emission lines. In the last catalogue of active galaxies Veron-Cetty & Veron (1996) compiled 220 BL Lac objects. BL Lac objects are violently variable not only in the optical (see Takalo et al. 1996 for the monitoring group: Florid group (Pica et al. 1980), Hamburg group (Schramm et al. 1994), Chinese group (Xie et al. 1994), Tuorla observation (Sillanpaa et al. 1991; Takalo et al. 1992) but also in the radio, infrared, UV, X-ray, and $\gamma$-ray bands (Tornikoski et al. 1994; Bregman et al. 1990; Soifer & Neugebauer 1980; Impey et al. 1984; Holmes et al. 1984; Urry 1984; Thompson et al. 1995, 1996). It has been shown that the spectral indices of some objects change with the brightness of the source; generally the spectrum becomes flat while the source brightens (Morini et al. 1986; George et al. 1988; Fan 1993). In contrast, the BL Lac object 3C 66A shows a different behaviour (De Diego et al. 1997).

Polarization, which is a characteristic of BL Lac objects, is variable. The maximum optical polarization has been found to be associated with both the beaming factor (Fan et al. 1997) and the largest optical variation (Fan & Feng 1998). Infrared polarization has been reported to be correlated with infrared spectral indices (Impey et al. 1982). Wills et al. (1992) found that polarization is associated with the core-dominance parameter. So, the extreme property of polarization in blazars could be explained as the effect of the relativistic model.

BL Lacertae (2200+420), the archetype of its class, lies in a giant elliptical galaxy at a redshift of $\sim$0.07 (Oke & Gunn 1974; Thuan et al. 1975). It was thought to be a "variable star'' by Schmit (1968) and was identified with the unusual radio source VRO 42.22.01 by MacLeod & Andrew (1968). The detection of variability in the radio flux (Biraud & Veron 1968; Gower 1969) and the featureless spectrum (Oke et al. 1969; DuPuy et al. 1969) resulted in the suggestion that it is a BL Lac object (Strittmatter et al. 1972). Its early properties were reviewed by Stein et al. (1976) and Miller (1978). It is one of the best-studied objects in the optical and radio bands. Superluminal radio components have been observed (Vermeulen & Cohen 1994; Fan et al. 1996). The optical and radio emissions are both variable and polarized, and the radio and optical polarizations are correlated (Sitko et al. 1985). The maximum polarizations obtained in the radio, infrared and optical bands are $P_{\rm Rad}=10.0\%$ (Gabuzda et al. 1989), $P_{\rm IR}=15.1\%$ (Impey et al. 1984), and $P_{\rm Opt.}=23\%$ (Angel & Stockman 1980) respectively. Recently, observations taken with EGRET on the Compton Gamma Ray Observatory (CGRO) between 1995 January 24 and 1995 February 14 indicate a flux of $(40\ \pm\ 12)\ 10^{ -8}$ photon/cm²/s above 100 MeV (Catanese et al. 1997), but there is no evidence of gamma rays in Whipple observations (Kerrick et al. 1995; Quinn et al. 1995). BL Lac has been observed in the optical bands for about 100 years (Fan et al. 1998). Webb et al. (1988) constructed its historic optical light curve. Very recently we have discussed its periodicity and found a 14-year period from the B band light curve (Fan et al. 1998). BL Lacertae has been observed in the infrared since the beginning of the 1970s, but no long-term infrared properties have been discussed in the literature. In this paper, we will mainly study the variability and discuss the properties in the infrared. The paper has been arranged as follows: in Sect. 2, we give the bibliography of the data; in Sect. 3, we discuss them.