In binary stars of short period, axial rotation of the components tends to be synchronized with the orbital revolution. Rotation of B. and A stars is therefore slowed down, while for the later-type stars, it is accelerated. This latter fact probably contributes to the phenomenon of the RS CVn stars. Notable deviations from synchronism among short-period systems are probably related to mass transfer between the components.

Binary stars play a double role in astrophysics. On the one hand, they reveal properties and structures that would otherwise be almost unobservable, and therefore help considerably to understand the character and evolution of single stars. On the other hand, they create objects and phenomena that otherwise would not exist at all, thereby actually creating more problems than they help to solve. I think that the first aspect makes the binary stars extremely useful; the other aspect makes them extremely exciting and interesting. This dual role also appears in connection with rotation. The “revealing” property is best documented by the fact that the phenomenon of stellar rotation was discovered in binary stars, through the “rotation effect” distorting the radial velocity curve of an eclipsed star. A more recent example is the probable identification of a star considerably flattened and distorted by differential rotation, in agreement with the theoretical models advanced by Bodenheimer and Ostriker (1970). The star is the secondary component of BM Orionis, which is the faintest of the four stars forming the famous Orion Trapezium. BM Orionis was once suspected of harboring a black hole, since during the apparently total eclipse of the primary B3 star, is spectrum remained visible and no other spectrum could be detected (Doremus, 1970). A higher dispersion of our Lick spectra permitted us (Popper and Plavec, 1976) to detect weak lines of the secondary star, which is most likely a late A giant still contracting to the main sequence. The flat bottom of the light curve at the primary eclipse is best explained by a model first suggested by Hall (1971), involving a disk-shaped, differentially rotating star. It would be difficult to recognize the shape of such a star if it were not member of an eclipsing system.