Infrared, radio and optical variability of the BL Lacertae object 2007+777

We present infrared observations at wavelengths of 60 and 100 mu m and cont emporaneous radio (five frequencies) and optical (R band) monitoring of the BL Lac object 2007+777. During a three-week campaign with the ISO1 spacecr aft, the VLA(2), the Effelsberg 100m radio telescope, and optical telescope s in Heidelberg and on Calar Alto, Spain(3), we observed variability on tim e scales of a few days at 100 mu m, at radio, and at optical frequencies. W e discuss briefly several generic models which could account for our observ ations. The attempt at explaining the results by an intrinsic mechanism req uires an extremely small source angular size of the order of a few mu as(4) , which can be reconciled with the 10(12) K inverse Compton limit only when the Doppler factor of the bulk flow is about 40. The general behavior of t he variability of the BL Lac object 2007+777 found in this IR-radio campaig n follows the expectations of a shock-in-jet model, and we argue that this is the most likely explanation of our observations.