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1. Introduction

Since 1992 several observational campaigns searching for rapid variability of southern extragalactic radio sources have been carried out at the Instituto Argentino de Radioastronomıa (IAR) (Romero et al. 1994, 1995a; Romero & Combi 1995). These campaigns were conceived with the aim of gathering a set as complete as possible of radio variability data of southern active galactic nuclei (AGNs) on timescales ranging from hours to months at relatively low radio frequencies (tex2html_wrap_inline1083). Together with the abundant data concerning the behaviour of northern sources (e.g. Quirrenbach et al. 1992) they can provide elements for an all-sky statistics of rapid radio variability of AGNs. In this paper we present results of new observations of the sources PKS 1610-771, PKS 1830-211, and the central region of Cen A, with different temporal resolutions.

PKS 1610-771 is a strong (tex2html_wrap_inline1091), flat-spectrum (tex2html_wrap_inline1093, tex2html_wrap_inline1095) source with a steep optical spectrum (Hunstead & Murdoch 1980). It has been classified as a QSO by Véron-Cetty & Véron (1996). The object, with a redshift z = 1.710, has been observed at high angular resolution by the Southern Hemisphere VLBI Experiment (Preston et al. 1989). These observations indicate a 3.8 Jy elliptical Gaussian core elongated 10 mas along a position angle of tex2html_wrap_inline1099, and a 1.4 Jy circular Gaussian halo of 25 mas in radius at 2.3 GHz. The source, consequently, seems to be compact with no visible structure over the 10 mas. As far as we know there are no reports about radio variability for this object until now.

PKS 1830-211 is a strong, flat-spectrum source with a double structure at scales of 1 arcsecond. VLBI observations have shown the presence of an Einstein ring and two almost identical bright spots, which have been interpreted as images of a background lensed object (namely a QSO). Owing to its gravitationally lensed nature this system has call much attention in recent years (see, for instance, Jauncey et al. 1991; Nair et al. 1993; van Ommen et al. 1995). The total flux density at 1.4 GHz is tex2html_wrap_inline1103 (Pramesh Rao & Subrahmanyan 1988). Flux density variability has been found at 2.3 and 8.4 GHz by Lovell et al. (1996). At the latter frequency the source evolved from a flux density value of tex2html_wrap_inline1105 in late 1990 to a peak of tex2html_wrap_inline1107 in early 1992, and then decreasing up to tex2html_wrap_inline1109. Variability information about this object is important in order to constrain the proposed lens models (e.g. van Ommen et al. 1995).

The third source of our sample, Cen A, is the nearest active galaxy: its distance is tex2html_wrap_inline1111 (Hui et al. 1993). It is a complex and extremely powerful source in the southern sky, covering an area of about tex2html_wrap_inline1113. The large scale radio structure has been investigated by Cooper et al. (1965), Junkes et al. (1993) and Combi & Romero (1996), among others. The major radio components of the source are the Northern and Southern Giant Lobes, the Northern Middle Lobe, the Double Inner Lobes, the jet (also visible at X-rays), and the mas-core (see composed maps in Meier et al. 1989). A weak counterjet has been recently discovered with a global (SHEVE + VLBA) array (Jones et al. 1996). The inner radio structure was studied in detail by Burns et al. (1983) with the VLA. Very rapid variability of the central regions of Cen A at mm-wavelengths has been reported by Kellerman (1974) and Kaufmann & Raffaelli (1979), and variability over larger timescales was detected by Abraham et al. (1982), Botti & Abraham (1993) and Abraham (1996), among others. Several authors have suggested that Cen A may harbour a misdirected BL Lac object (e.g. Bailey et al. 1986; Morganti et al. 1992). In such a case variability at cm-wavelengths might be produced in the inner region of the source over relatively short timescales (may be months).

In the next section we shall briefly describe our observations of the selected objects. The following sections are devoted to the results and their discussion.


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