Multifrequency observations of blazars. IV - The variability of the radio to ultraviolet continuum

The results of analysis of the variability properties of the high-frequency radio to UV continua of a sample of nine blazars are presented. The IR and optical variations of the three sources for which detailed comparisons can be made between the light curves obtained at the various wavebands are found to be well correlated. A strong correlation is observed beween the near-IR flux levels and the near-IR spectral slopes of the BL Lac objects, in the sense that the spectra are steeper when the sources are fainter. A similar correlation is observed to hold in optically violent variable (OVV) quasars during large-amplitude flare events; in general, however, the spectra of the OVV quasars exhibit no significant correlation between flux level and spectral slope. An additional nonvariable 1-5 micron component may be present in the near-IR spectra of OVV quasars