Contact Binaries with Additional Components.I. The Extant Data
  We have attempted to establish an observational evidence for presence of distant companions which may have acquired and/or absorbed the angular momentum during evolution of multiple systems thus facilitating or enabling formation of contact binaries. In this preliminary investigation we use several techniques (some of them distance-independent) and mostly disregard detection biases of individual techniques in an attempt to establish a lower limit to the frequency of triple systems. While the whole sample of 151 contact binary stars brighter than Vmax = 10 mag. gives a firm lower limit of 42+/-5%, the corresponding number for the much better observed Northern-sky sub-sample is 59+/-8%. These estimates indicate that most contact binary stars exist in multiple systems. 
Theodor Pribulla, Slavek M. Rucinski