3 EPOCH VLBI OBSERVATIONS AT 18 CM OF LOW-FREQUENCY VARIABLE SOURCES

We present third epoch VLBI observations at 1.67 GHz of a sample of 20 Low Frequency Variable Sources. The sources have been observed with a global array of 11 radio telescopes, recording in MK 2 mode, with a r esolution of similar to 3 mas. We also present an analysis of possible variations in the fringe visibilities between an earlier epoch (1981. 8) and the present epoch (1987.9), and compare these changes with thos e observed between the two epochs 1980.1 and 1981.8. Significant varia tions have been found in 17 out of 20 sources. In a number of cases th e changes are clearly due to flux density variations in the most compa ct component. Several other cases appear to be due to either an increa se in the angular size of a single component or a change in the angula r separation between well defined components. Some sources show superl uminal motions much like those usually observed at higher frequencies, and some new candidates for superluminal motions are identified. We h ave also used 408 MHz monitoring data, taken in the period of the VLBI observations, to derive an estimate of the Doppler factors necessary to account for the low frequency variability, and we compare these val ues with the Doppler factors calculated from the proper motions detect ed in our VLBI observations. This analysis confirms that two distinct phenomena, intrinsic superluminal beaming and extrinsic scintillation, are responsible for the low frequency variability seen in compact ext ragalactic radio sources.