Unusual radio variability in the BL Lacertae object 0235+164

We present radio observations at three frequencies and contemporaneous opti cal monitoring of the peculiar BL Lac object AO 0235+164. During a three-we ek campaign with the VLA we observed intraday variability in this source an d found a distinct peak which can be identified throughout the radio freque ncies and tentatively connected to the R-band variations. This event is cha racterized by unusual properties: its strength increases, and its duration decreases with wavelength; and it peaks earlier at 20 cm than at 3.6 and 6 cm. We discuss several generic models (a "standard" shock-in-jet model, a p recessing beam, free-free-absorption in a foreground screen, interstellar s cattering, and gravitational microlensing), and explore whether they can ac count for our observations. Most attempts at explaining the data on 0235+16 4 require an extremely small source size, which can be reconciled with the 10(12) K inverse Compton limit only when the Doppler factor of the bulk flo w is of order 100. However, none of the models is completely satisfactory, and we suggest that the observed variability is due to a superposition of i ntrinsic and propagation effects.