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The blazar AO 0235+164 was claimed to show a quasi-periodic behaviour in theradio and optical bands. Moreover, an extra emission component contributing tothe UV and soft X-ray flux was detected, whose nature is not yet clear. Apredicted optical outburst was observed in late 2006/early 2007. We herepresent the radio-to-optical WEBT light curves during the outburst, togetherwith UV data acquired by Swift in the same period. We found the opticaloutburst to be as strong as the big outbursts of the past: starting from lateSeptember 2006, a brightness increase of 5 mag led to the outburst peak inFebruary 19-21, 2007. We also observed an outburst at mm and then at cmwavelengths, with an increasing time delay going toward lower frequenciesduring the rising phase. Cross-correlation analysis indicates that the 1 mm and37 GHz flux variations lagged behind the R-band ones by about 3 weeks and 2months, respectively. These short time delays suggest that the correspondingjet emitting regions are only slightly separated and/or misaligned. Incontrast, during the outburst decreasing phase the flux faded contemporaneouslyat all cm wavelengths. This abrupt change in the emission behaviour may suggestthe presence of some shutdown mechanism of intrinsic or geometric nature. Thebehaviour of the UV flux closely follows the optical and near-IR one. Byseparating the synchrotron and extra component contributions to the UV flux, wefound that they correlate, which suggests that the two emissions have a commonorigin.

Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006--2007 outburst