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We present visible and near infrared observations of the blazar OJ287 taken during a flare of nearly two magnitudes amplitude in V, which peaked on December 6th 1993 at V=14.5 (Kidger et al., 1993, IAUC 5909). Simultaneous visible (BVRI) and near infrared (JHK) photometry were taken on two nights during the decline phase of this outburst. The observations reveal considerable variations in the level of activity during the flare: at maximum very rapid variations were seen (several tens of percent in less than half a hour); whilst a very low amplitude of variation was generally observed pre- and post-maximum.
Our data show significant variations in the colours and hence spectral slope in the infrared on timescales of hours, but very little variability in the visible, suggesting a high frequency cut-off in the variations of the continuum spectral slope at close to 1 micron. The size and time scale of the variations are inconsistent with a shocked-jet model and require highly variable synchrotron emission from small regions of the accretion disc.
