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Prompted by a high optical state in September 2007, the Whole Earth BlazarTelescope (WEBT) consortium organized an intensive optical, near-IR (JHK) andradio observing campaign on the intermediate BL Lac object 3C 66A throughoutthe fall and winter of 2007 -- 2008. The source remained in a high opticalstate throughout the observing period and exhibited several bright flares ontime scales of ~ 10 days. This included an exceptional outburst aroundSeptember 15 - 20, 2007, reaching a peak brightness at R ~ 13.4. Our campaignrevealed microvariability with flux changes up to |dR/dt| ~ 0.02 mag/hr. Ourobservations do not reveal evidence for systematic spectral variability orspectral lags. We infer a value of the magnetic field in the emission region ofB ~ 19 e_B^{2/7} \tau_h^{-6/7} D_1^{13/7} G. From the lack of systematicspectral variability, we can derive an upper limit on the Doppler factor, D <=28 \tau_h^{-1/8} e_B^{3/16}. This is in agreement with superluminal motionmeasurements of \beta_{app} \le 27 and argues against models with very highLorentz factors of \Gamma > 50, required for a one-zone SSC interpretation ofsome high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.

The WEBT Campaign on the Intermediate BL Lac Object 3C66A in 2007-2008