1 Introduction

BL Lac objects are a special subclass of active galactic nuclei (AGNs), which show some extreme properties: rapid and large variability, high and variable polarization, no or only weak emission lines in its classical definition.

BL Lac objects are variable not only in the optical band, but also in radio, infrared, X-ray, and even $\gamma$-ray bands. Some BL Lac objects show strongly correlated variation between radio and optical emissions with some delay (e.g. Tornikoski et al. 1994). Some BL Lac objects show that the spectral indices change with the brightness of the sources (Bertaud et al. 1973; Brown et al. 1989; Fan 1993), generally, the spectrum flattens when the source brightens, but different phenomenon has also been observed, from 3C 66A for instance (see De Diego et al. 1997).

The nature of AGNs is still an open problem; the study of AGNs variability can yield valuable information about their nature, and the implications for quasars modeling are extremely important (Blandford 1996). From the telescopes in China (the optical telescopes: 1-m telescope at Yunnan Observatory, the 1.56-m telescope at Shanghai Observatory, and the 2.16-m telescope at Beijing Observatory; and the 1.26 m infrared telescope at Beijing observatory), we have monitored dozens of AGNs, including BL Lac objects, quasars, and Seyfert galaxies (Xie et al. 1987, 1988a,b, 1990, 1991, 1992, 1994; Fan et al. 1997; Bai et al. 1998; Xie et al. 1998).

OJ 287 (VRO 20.08.1) was discovered in radio observation by Dickel et al. (1967) and in the optical band by Thompson et al. (1968). Spectroscopic observations of Miller et al. (1978) showed a weak $\rm [O_{III}]$ spectral feature with a redshift of 0.306, which was confirmed by later observation of Sitko & Junkkarinen (1985) and Stickel et al. (1989). Early observations indicated that it was variable in the radio (Blake 1970) and optical bands (Kinmin & Conkin 1971). It was observed to show only continuum emission (Adam et al. 1972) and high linear polarization (Kinman & Conkin 1971; Nordsiek 1972). It has been observed extensively since.

OJ 287 is one of the very few AGNs for which continuous light curve over more than one hundred years has been observed. Its observational properties from radio to X-ray have been reviewed by Takalo (1994). The light curve in optical band shows the strong signature of an outburst that occurs with a period of 12 years. Models including binary black hole can explain these variations (Sillanpaa et al. 1988a). The outburst predicted to occur in the fall of 1994 confirmed the 12-year period (Sillanpaa et al. 1996b,c).

Besides the long-term large outburst, OJ 287 has also shown some interesting fluctuations of brightness on times ranging from less than 1 hour (Visvanathan et al. 1973; Veron & Veron 1975; Carrasco et al. 1985) to about a week (Kinman et al. 1974). It is one of the objects in our optical monitoring program (see Xie et al. 1994; Bai et al. 1998).

Large infrared variations have been seen in the source. Variations of 0.3 to 0.5 magnitudes in the near-infrared bands over a time scale of one day have been reported during its outburst by Holmes et al. (1984a) and during its low state by Wolstencroft et al. (1982) and Impey et al. (1984). Lorenzetti et al. (1989) reported variations of $0\hbox{$.\!\!^{\rm m}$}3$ over a time scale of 3 hours and $0\hbox{$.\!\!^{\rm m}$}5$ over a day and $\langle J-H \rangle$ = 0.83, $\langle H-K \rangle$ = 0.91. As reported in the optical band (Takalo & Sillanpaa 1989), a correlation between color index and magnitude has been found in the infrared band: Gear et al. (1986a) found a correlation between the infrared spectral index and the J band flux from data covering the period February 1983 to February 1986 after the 1983 outburst. Kidger et al. (1994) and Zhang & Xie (1996) also found the correlation. But no such significant correlation was found in the data of 1986 February through 1987 December (Lorenzetti et al. 1989). In November 1995, it was detected in our observation program at Xinglong station of Beijing Observatory (see Xie et al. 1998 for detail).

In this paper, we will present both historical and new data in the optical and infrared bands, and deal with them by means of the DCF method. The paper has been arranged as follows: In Sect. 2, we will present the observations; in Sect. 3, the correlation analysis method (DCF); in Sect. 4, we will discuss our results; and in Sect. 5, we will give a brief conclusion.