  Starting from December 2004, a program for the monitoring of intraday
variable sources at a frequency of 5 GHz was performed at the Urumqi
Observatory. The analysis of the variability characteristics of the
flat-spectrum radio source AO 0235+164 revealed the existence of an annual
cycle in the variability amplitude. This appears to correlate with the solar
elongation of the source. A thorough analysis of the results of the MASIV IDV
survey --- which provides the variability characteristics of a large sample of
compact radio sources --- confirms that there is a small but detectable
component of the observed fractional modulation which increases with decreasing
solar elongation. We discuss the hypothesis that the phenomenon is related to
interplanetary scintillation.
