Published online by Cambridge University Press: 03 August 2017
Observations of short term flux variations provide one of the few means of determining the smallest scale sizes in compact radio sources. The fastest variations are generally found in BL Lac-type objects, OJ287 being one of the best studied. In 1985 Valtaoja et al. (Nature 314, 148) found evidence for variations at a level of (typically) a few % with a period of in OJ287 at frequencies of 22 and 37 GHz. [Such periodic variability was, however, not detected at 5 GHz during one of their observations by Dreher et al. (Nature 320, 239, 1986)].