2 The sample

From the Metsähovi radio monitoring list we selected objects for which the radio light curves were extensive enough for the analysis, i.e. there was enough data for an attempt to apply the total radio flux density decomposition to separate flares (see Sect. 3.2), and for which sufficient amount of recent optical data was found. Details of the Metsähovi radio monitoring program and the radio data are described in Salonen et al. (1987), Teräsranta et al. (1992) and Teräsranta et al. (1998). Our sample consists of 20 AGN, of which 9 are quasars and 11 BL Lacs.

The objects are classified as "BLO''$\rm ~~=BL$ Lac object, "HPQ'' $\rm ~~=High$ optical Polarization Quasar, "LPQ'' $\rm ~~=Low$ optical Polarization Quasar. The quasar / BL Lac classifications are adopted from Véron-Cetty & Véron (2001).

The optical data is collected from several articles. References are given in Table 1. The optical data is from all optical frequency bands (U, B, V, R and I). The data was first converted to the optical V-band by using colour information calculated from the data, because the colours may not be constant with time. The calculated colours were checked with results given in Véron-Cetty & Véron (2001), and if there were not enough data points for calculation, the colours from the catalogue were chosen instead. Transformations from magnitudes to flux densities were done according to Mead et al. (1990).