Abstract
An analysis has been made of all the observations available of Ambartsumian's trapezia, with the aim of establishing the kinematic properties of this interesting class of multiple stars. It was found that there is no evidence for a systematic expansion in any of the 46 trapezia that were studied. In particular, the Orion Trapezium, the best documented case, shows no expansion. However, a sizeable fraction of the trapezia show one or two stars with relative motions. A statistical test and a proper motion test have been applied to establish membership of these stars in their trapezia. Some of these stars have transverse velocities larger than 30 km sec1. This behaviour is important in understanding the dynamics of trapezia.
