The Frequency of Binaries among Metalmc-Line Stars.

The question of what fraction of the metallic-line (Am) stars are spectroscopic binaries is treated by an intensive study of a random sample of 25 such stars. It is found that only 3 of these have constant velocities. Of the remaining 22, orbital elements are now known for 18 and periods (only) for 2 more. Estimates of the fraction of the binaries which would be undetectable because their velocity amplitudes are too small easily account for the 3 onstant-velocity stars, and it is concluded that 100 per cent of the Am stars are spectroscopic binanes. There is nothing obviously unusual about the secondary stars such as to suggest that they are causing the abnormal appearance of the spectra of the Am stars. The composite nature of these stars does not seem to account for their location about 0.7 mag. above the main sequence in a color-absolute-magnitude diagram. There is a rough correlation between the discrepancy in spectral-type classification from various criteria and the ultraviolet deficiency shown by essentially all Am stars. On the supposition that in close binaries the axes of rotation and revolution should be approximately parallel, we can determine that the low projected rotational velocities (about one-third normal) of Am stars are not due to a selective aspect effect. However, in the close binaries the rotational velocities have been modified to synchronize with the orbital motion: in binaries with periods less than 1.8 days the rotation is increased, and in ones with slightly larger periods (1.8 .5 days) it is slowed down. Two stars (32 Vir and u Oph) have mass functions or mass ratios which indicate that their primaries or secondaries violate the mass-luminosity relation for stars near the main sequence. Furthermore, these two stars have line strengths that vary with phase, indicating gaseous streams, although both are far from being close binaries. Orbital elements are given for eleven new binaries.