Optical Monitoring of BL Lacertae Object OJ 287: A 40 Day Period?

We present the results of our optical monitoring of the BL Lacertae object OJ287 during the first half of 2005. The source did not show large-amplitudevariations during this period and was in a relatively quiescent state. Apossible period of 40 days was derived from its light curves in three BATCwavebands. A bluer-when-brighter chromatism was discovered, which is differentfrom the extremely stable color during the outburst in 1994--96. The differentcolor behaviors imply different variation mechanisms in the two states. We thenre-visited the optical data on OJ 287 from the OJ-94 project and found as wella probable period of 40 days in its optical variability during the late-1994outburst. The results suggest that two components contribute to the variabilityof OJ 287 during its outburst state. The first component is the normal {\slblazar} variation. This component has an amplitude similar to that of thequiescent state and also may share a similar periodicity. The second componentcan be taken as a `low-frequency modulation' to the first component. It may beinduced by the interaction of the assumed binary black holes in the center ofthis object. The 40-day period may be related to the helical structure of themagnetic field at the base of the jet, or to the orbital motion close to thecentral primary black hole.Comment: 31 pages, 8 figures, accepted by A