There is a very striking relation between the eccentricity and the period of a system; in general the binaries of longest period have the greatest eccentricities.
The relation applies not only to the visual but to the spectroscopic binaries; these, having shorter periods than the visual binaries, have generally quite small eccentricities.
Up to 1905, 140 spectroscopic binaries had been discovered; a list of these is given in the Lick Observatory Bulletin, no.
A continuous gradation can be traced from the most widely separated visual binaries, whose periods are many thousand years, to spectroscopic binaries, Algol and # Lyrae variables, whose periods are a few hours and whose components may even be in contact, and from these to dumb-bell shaped stars and finally to ordinary single stars.
Lvi.; the orbits of the principal binaries are discussed in T.