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Up: Monitoring of AO

1. Introduction

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 tex2html_wrap_inline1050-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 and references 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 tex2html_wrap_inline1052 (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).

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