Infrared properties of OJ 287

The paper presents the first evidence for the long-term near-infrared brightness, colour and spectral variability in the BL Lac object OJ 287. It had an infrared outburst of large amplitude during the beginning of 1983. The J-K colour index increased linearly, and infrared spectrum also linearly steepened during the years 1975-1990. After then, both the J-K colour and spectral index began to recover. Both the J-K colour and the near-infrared spectral index shows also a strong correlation with the flux intensity, in the sense that when OJ 287 is fainter, J-K is larger, and the near-infrared spectrum steeper. These properties show a very important similarity with the optical data of OJ 287 from the same time interval (Takalo & Sillanpaa 1989). Our results argue that a single variable synchrotron source might be responsible for the photometric behaviour of OJ 287 in the near-infrared and optical regions.