Abstract PKS 2155−304 is a well studied BL Lac object in the southern sky. The historical optical data during different periods have been collected and compiled. Light curves spanning 35 yr have been constructed. The R-band light curve has been analyzed by means of three methods: the epoch folding method, the Jurkevich method and the discrete correlation function method. It is derived that there is an evident periodic component of 317 d (i.e. 0.87 yr) superposed on a long-term trend with large-amplitude variation in the light curve. The variability of this source is accompanied by a slight color variation, and the brightness and color index are correlated with each other. On a long-term time scale, PKS 2155−304 exhibits a tendency of bluer-when-brighter, which means the spectrum becomes flatter when the source brightens.
Keywords BL Lacertae objects: general — BL Lacertae objects: individual (PKS 2155−304) — galaxies: active — method: statistical
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