SIMBAD references

Observations with the Effelsberg 100-m telescope and the Very Large Array (VLA) have shown that the radio emission of some quasars and BL Lacertae objects varies on the time scale of only one day. Simultaneous optical variations have been detected with a 71-cm telescope of the Landessternwarte Heidelberg and the Calar Alto 2.2-m telescope. These observations pose serious problems to the generally accepted "standard model" of quasars; it has not yet been possible to explain them completely. More observations to characterize the properties of rapid variability in quasars are urgently needed. Regular monitoring of a few relatively bright objects might be a challenging but rewarding project for observers with telescopes of moderate size and CCD equipment.