The problem of rejecting harmonic disturbances occurring at the input of a linear controller in a stable loop is addressed. The task is to recover regulation of the plant output, using only information corrupted by harmonic noise of unknown frequency. The solution is provided by an adaptive external model (a device placed outside the stabilizing loop), which possesses a hybrid system structure due to the specific form of the associated certainty-equivalence solution. The geometric characterization of the solution is emphasized throughout the paper, and its relation with the internal model principle is discussed. The important issue of convergence in absence of persistence of excitation is addressed in full. Experimental results validate the proposed approach. (c) 2006 Elsevier Ltd. All rights reserved.