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

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