Such, for instance, are the systems of _Zeta Cancri_ and _Epsilon Lyræ_. In the former system the closer components revolve rapidly about their center of gravity in a period of about sixty years, while the remote companion shows irregularities in its motion that indicate that it is revolving about a dark body in a period of seventeen and a half years, while the two together are revolving very slowly in a period of six or seven centuries, about a common center of gravity with the first pair in a retrograde direction.