The BL Lac object AO 0235+164 is one of the most complex and interesting
extragalactic sources. It has shown variability in all observed
frequencies, from -rays (von Montigny et al. 1995)
to radio bands
(e.g. Teräsranta et al. 1992).
The variability amplitude in the optical bands
has spanned more than five magnitudes (Webb & Smith 1989).
These variations are
often seen as fast, very intensive flares (Webb & Smith 1989
therein). Schramm et al. (1994) observed a 1.60 mag brightening in 47.5
hours in February 1989.
AO 0235+164 is characterized by a very steep optical spectrum,
the observed range in
the spectral index being (Smith et al. 1987).
Polarization measurements have revealed a large and rapid variability
(Impey et al. 1982; Smith et al. 1987)
and values up to 44%
have been detected (Impey et al. 1982).
This source has been claimed to display
simultaneous variations in optical and radio bands (e.g Rieke et al.
1976; Balonek & Dent 1980).
In reality, AO 0235+164 is a system consisting of several objects with
three different redshift values. These are: the BL Lac itself, with z=0.94,
a pair of galaxies at z=0.524, and a so far unknown system with z=0.851
(see Nilsson et al. 1996
and Burbidge et al. 1996 for a detailed description of the system).
Because of the intervening galaxies and the monochromatic variability,
AO 0235+164 has been considered as one of the best candidates for BL Lac
objects in which the variability is caused by microlensing
(Stickel et al. 1988; Rabbette et al. 1996).