The light-changes of double stars are commonly of a fitful and indecisive kind. They may affect one or both members of stationary pairs; but visibly revolving stars, as a rule, conspire to vary, if they vary at all. The alternating fluctuations of γ Virginis, discoverable only by close attention to the swaying balance of lustre between the components, are in this respect typical. Each may be described as normally of the third magnitude, and each in turn declines by about half a magnitude and recovers within a few days, yet so that the general preponderance during a cycle of several years, remains to the same star. The existence of this double periodicity was recognised in 1851 by M. Otto Struve, who, however, despaired of investigating it with success in a latitude where the stars in question never rise more than 30° above the horizon.

Their circulation is in the most eccentric of ascertained stellar orbits (see fig. 30). The ellipse traversed by γ Virginis in 180 years is, in fact, proportionately somewhat narrower than the path round the sun of Encke's comet, so that the stars will in 1926 be separated by fully seventeen times the interval of space between them in 1836, when they merged into a single telescopic object. Their inequalities of light seem to have developed as they approached each other; at least, they first began to be noticed by Struve in 1818, and they at present tend to become obliterated, whether to revive with regained proximity towards the close of the twentieth century, future observations must decide.