2 The selected sources

We selected a sample of sources in the mid-southern hemisphere, with $-30^{\circ} \le \delta \le +30^{\circ}$.Since the aims of the present work are to monitor the variability in the milliarcsecond scale radio structure, and to collect more sources in the mid-southern hemisphere to supplement the above-mentioned VLBI surveys, we included in our sample both core dominated flat spectrum sources and radio sources which had been suggested as superluminal candidates by Shen et al. (1997 and 1998). Our sample includes:

a) seven sources observed and studied in November 1992 and May 1993, which were chosen for monitoring purposes. These are 0420-014, 1034-293, 1334-127, 1504-166, 1730-130, 2243-123, 2345-167;

b) sources with correlated flux density $S_{\rm 2.3 GHz} \ge$ 0.5 Jy (Preston et al. 1985) and with flat spectra (i.e. $\alpha \sim -0.5$).

20 extragalactic radio sources which met the above criteria are selected for the observation. The source DA193 (0552+398) is chosen as amplitude calibrator. The sample is listed in Table 1: Col. 1 - IAU name; Cols. 2 and 3 - positions for the epoch J2000; Col. 4 - optical identification (Q: Quasar , Bl: Blazar); Col. 5 - redshift (Veron-Cetty & Veron 1995; Preston et al. 1985); Col. 6 - total flux density at 5 GHz: the values with asterisks were measured at Hartebeesthoek during the experiment and the remaining ones are listed in the Parkes Catalogue (Wright & Otrupcek 1990).

    

Table 1: The sources for the 5 GHz VLBI observation

Table 2: VLBI telescopes and their characteristics at 5 GHz