The main results from this monitoring are shown in Figs. 1-12, which show the observed light curves. We have transformed the optical and infrared magnitudes to flux units shown in some of these figures, using the calibration given by Mead et al. (1988). Figure 1 displays the historical light curve with the new data included. As can be seen from this figure (and Fig. 2) the outbursts were detected close to the predicted times (see Sillanpää et al. 1996a,b for details). Using these new outbursts together with the historical data we can now find an exact period for the outbursts of 11.86 years (see also Sillanpää 1999; Valtaoja et al. 2000). In determining this period we have taken into account the fact that there are always two outbursts, separated by 1.1 years. These optical outbursts have been explained by the "old'' binary black hole model (Sillanpää et al. 1988), with a "new'' version of this model (Lehto & Valtonen 1996; Sundelius et al. 1997; Valtonen & Lehto 1997), with a precessing jet model (Katz 1997), and with a binary curved-jet model (Villata et al. 1998). All these models can explain (at least) part of the observations, but there are problems with the combined data, especially with the radio observations. The comparison between optical and radio light curves (e.g. Figs. 2, 9 and 12) observed during the latest outbursts show clearly that during the first optical flare the radio bands showed minimum flux (see Valtaoja et al. 2000).

Figure 2 shows the observed optical light curves, displaying clearly the outbursts in November 1994 and end of December 1995 (Arimoto et al. 1997; Fan et al. 1998b; Kidger et al. 1995; Sillanpää et al. 1996a,b). The separation between these two outbursts is 1.1 years (Sillanpää et al. 1996b; Valtonen & Lehto 1997), which is very similar to the one observed in 1972/73 and 1983/84 (Sillanpää et al. 1988). As can be seen from the figures the shape of these two outbursts is very different, the first one being fairly sharp and the second one much broader. In the first case we can also define the time of the peak quite accurately (V=13.92 on November 10th 1994). But during the second outburst the timing of the peak is much more difficult because of the broadness of the outburst and the small gaps in the data. Since late 1996 the brightness of OJ 287 has declined quite rapidly, and the brightness is now back at the preoutburst level (see also Pietilä et al. 1999).

It is also apparent from the data that OJ 287 is varying all the time, we cannot define any quiescent level from these data. The light curves can be characterized by flaring activity with time scales from days to about a week and amplitudes up to one magnitude (see Lehto 1994). An example of this kind of behaviour is shown in Fig. 3, where we show the V-band light curve with increasing time resolution during the first outburst. The variability in different time scales can be clearly seen. On top of the flaring activity we see small amplitude flickering with a time scale from tens of minutes to hours (Dultzin-Hacyan et al. 1997; Jia et al. 1998). The behaviour is very similar in all optical bands (Hagen-Thorn et al. 1998; Lehto 1994; Sillanpää et al. 1996b). Most of these flares have very similar shape with a very fast increase in brightness and a slower declining phase. It is also evident that this flaring activity continues after the outbursts, but with smaller flare frequency and amplitude (see Fig. 2).

4.2 Optical polarisation

Linear optical polarisation light curves are shown in Figs. 4 and 5. The shown data are all nightly mean values. As can be seen from Fig. 4 the polarisation showed large random variability. The behaviour is seen to be very similar in all five bands. Noticeable is the fairly low polarisation level observed during the outbursts, especially the November 1994 outburst. Similar behaviour was observed also during the 1983/84 outburst (Takalo 1994 and references therein). The level of polarisation increased substantially (up to 25%) after the November outburst.

Also the polarisation position angle shows large variability. But it seems that the "average'' position angle rotated from the typical value of 90 degrees (e.g. Takalo 1994) to 150 degrees around the time of the first optical outburst (Fig. 5). This is similar value than the one observed during the previous outburst in 1983/84 (Smith et al. 1987). Note that in Fig. 5 we show the polarisation data without "correcting'' for the 180 degree ambiguity.

4.3 Infrared data

We have infrared data mostly from the first observing period 1993-1994. The light curves are shown in Fig. 6. As can be seen from the figure the behaviour is very similar, with the flaring activity, to the one seen in optical bands. Small flares with an amplitude up to one magnitude are seen in time scales of a few days. The behaviour seems to be more active than what was observed earlier, before the outburst (Takalo et al. 1992). Also the first (November 1994) outburst is clearly seen in the infrared bands (see Kidger et al. 1995).

4.4 Radio data

The radio data is shown in Figs. 7-10. The most noticeable feature in these light curves are the low flux levels observed during most of this time. Observations taken during 1993-94 are among the faintest ever observed for OJ 287 (for comparison see Aller et al. 1992, 1999; Takalo et al. 1990; Teräsranta et al. 1998; Tornikoski et al. 1996). This is a very different behaviour when compared to the previous outburst, seen in 1983/84, when also the radio flux was in a high level at the time of the optical outbursts (Takalo 1994; Valtaoja et al. 1999), but without any noticeable radio outburst. A radio outburst was observed simultaneously with the second optical outburst in December 1995. The peak of this outburst was at the time of the optical peak. But the radio outburst was much faster than the optical outburst, which lasted almost an year (compare Fig. 2 with Figs. 7, 9 and 12). After the outburst the radio flux returned very rapidly to the level seen before the outburst. The flux level also remained at low levels, until 1998, when the radio flux started to increase again. Note, however, that the optical flux was still decreasing at this time. Small amplitude variations in time scales from days to weeks are seen throughout the monitoring period.

4.5 Radio polarisation

The observed radio polarisation and position angles are shown in Fig. 7. We show only the data at 8 GHz, because both 4.8 and 14 GHz show very similar behaviour. The radio polarisation behaviour was very similar to that observed in the optical bands. The observed polarisations gave very low values, like the ones observed during the 1983/84 outburst (Aller et al. 1999; Takalo 1994). The polarisation level stayed also fairly stable at about 2% level. The highest polarisation values were observed about one year before the outburst, again resembling to what was seen in 1983/84.

The position angle shows almost constant values at $\rm PA=90^{\circ}$ at the begining of this monitoring. But, a few months after the first optical outburst, the position angle turned to PA = 170 degrees, where it remained thereafter. This behaviour is similar to the position angle seen in the optical polarisation, but with the difference that the optical position angle turned to this value before the radio one, this is shown in Fig. 8. This similarity suggests, that the radio and optical emitting regions (or at least their magnetic field orientations) are related. The position angle in radio bands showed also small amplitude short term variability. Similar behaviour was seen during the 1983/84 outburst (Aller et al. 1999; Takalo 1994). Taking into account the 180 degree ambiguity in the position angle values, it seems thatnoticeable rotation is present in the radio position angle during the radio brightening towards the end of our observations. The level of the polarisation is low and stable at this time.

$\begin{figure} \par\epsfig{figure=h1844f12.eps,width=14.0cm,height=16.0cm,clip=}\par\end{figure}$

Figure 12: Comparison of the behaviour of OJ 287 at different wavelengths. Note different scales in the y-axis

4.6 UV data

After the brightening of OJ 287 in late 1994, we observed OJ 287 with the IUE. The observed light curve at 2550 Å is shown in Fig. 11. As can be seen, a small outburst is visible. The UV flux was lower by a factor of two than the maximum UV flux recorded during the previous outburst in 1983 (see Pian et al. 1996; Takalo 1994). This UV flare lasted only about a week. The spectrum ( $f \propto \nu^{- \alpha}$ ) was remarkably steep during these observations, with a slope of $\alpha$ = 1.8 in the entire IUE range (Pian et al. 1996).

4.7 X-ray and $\gamma$ -ray data

OJ 287 was observed with the Rossi X-ray Timing Explorer (RXTE) since 2 November 1996 to the end of the campaign reported here. The observations were carried out about once per week, with two-month gaps during the summer when the object was too close to the sun. Aside from a 3.6- $\sigma$ detection on 25 February 1997, OJ 287 was not detected by RXTE. The typical 2- $\sigma$ noise level in the 2 - 10 keV band was $8.4\ 10^{-13}$ erg cm^-2 s^-1. If we consider this to be the upper limit to the non-detections, we can state that the X-ray flux in this band was at least 6 times fainter than during the Nov. 1994 outburst ( $5.1\ 10^{-12}$ erg cm^-2 s^-1; Idesawa et al. 1997).

After the brightening of OJ 287 during autumn 1994, we received some ToO time for OJ 287 with the EGRET onboard the CGRO. EGRET observed OJ 287 for five days (November 10-15. 1994), getting a marginal detection at the 3.5 $\sigma$ level of $F=2.9\ 10^{-7} \rm photons/cm^2/s$ (see Shrader et al. 1996; Webb et al. 1996). This is a factor of three larger than the flux observed earlier, when OJ 287 was in a low optical state.