Abstract
We have acquired extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe-O8fp). The Balmer and HeI lines show strongly variable emission which is extremely reproducible on a well-determined 538-d period. Metal lines and HeII absorptions (including many selective emission lines, but excluding HeII λ 4686 Å emission) are nearly constant in line strength, but are variable in velocity. The radial-velocity variations establish a double-lined binary orbit with P_{orb} = 1542d, e = 0.44; by elimination, rotational modulation of a magnetically constrained plasma is left as by far the most likely `clock' underlying the 538-d changes. The implied rotation period shows that slow rotators can easily be hidden in the O-star population, with gaussian-like `turbulence' dominating the line widths.
