1 Introduction

OJ 287 is one of the best studied blazars (e.g. Takalo 1994). After constructing its historical light curve Sillanpää et al. (1988) found a periodicity in the strong optical outbursts of the object (with $P\approx11.6$ years) and predicted that the next large flare should occur in 1994. This prediction was based on a binary black hole model for OJ 287 (see also Lehto & Valtonen 1996). Verification of this prediction is one of the goals of the international OJ-94 project, during which the object has been intensively monitored at all available wavelengths since fall 1993 (Kidger et al. 1998). In November 1994 the brightness increase in the IR-optical and UV regions was observed, confirming the prediction (Sillanpää et al. 1996a). This is the first case when a predicted photometric behaviour of extragalactic object has been confirmed. Another large and expected flare was observed during December 1995 (Sillanpää et al. 1996b).

By current understanding of the blazar emission the most of it at lower frequencies (in radio bands perhaps all) is due to synchrotron radiation coming from the jet (e.g. Urry & Padovani 1996). The synchrotron nature of the emission is supported by the observed power-law type spectra and the strong and variable polarization (e.g. Efimov & Shakhovskoy 1996).

This paper is devoted to the analysis of the OJ 287 colour behaviour in IR-optical-UV region before and during the outburst.