Orbital inclinations of late B-type spectroscopic binaries

Information on the orbital inclinations j of late B-type spectroscopic binaries (SB's) with periods between 3 and 50 days is obtained from the masses M1 of their primary components, derived from uυbyβ photometry, and the values of M1 sin3 j. The cumulative distribution of j for a fairly complete sample of double-lined binaries (SB2's) with Hg–Mn primaries is consistent with that expected for random orientations of the orbital planes. The period-eccentricity relations for Hg–Mn SB's and normal, sharp-lined SB's do not differ significantly. Subsynchronous rotators occur among the components of Hg–Mn SB's (e.g. HR 266, ϰ Cnc, HR 4072, χ Lup and 74 Aqr) and superficially normal SB's (e.g. 64 Ori, HR 7338 and possibly HR 4892); the subsynchronous primary of HR 7338 is metal poor. The slow rotation of Hg–Mn stars is probably due to special initial conditions or to a substantial loss of angular momentum during contraction to the main sequence. The orbital periods of three of the SB2's with nonsynchronous Hg–Mn components (HR 266, AR Aur and 74 Aqr) are only about 4 days, and these systems may pose a difficulty for the hypothesis that the abundance anomalies are due to the separation of elements by diffusion in quiescent atmospheres.