The extraordinary optical outburst of ON 231 (W Com) in spring 1998

The BL Lac object ON 231 (1219+285, W Com) had an exceptional outburst phas e in April-May 1998 and reached the most luminous state, observed at least since the beginning of the century. In this paper our multiband photometric light curves and polarimetric measurements of this event are presented. In the maximum phase the magnitude of ON 231 was R(Cousins)=12.2 and the opti cal spectral distribution was the flattest ever observed. Our data indicate that the main features of this burst can be explained by the presence of s ome emission components having different polarisation states and variabilit y time scales ranging from 1 day to several months.