  We present 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 highly reproducible on a well-determined 538-d period. Metal
lines and HeII absorptions (including many selective emission lines but
excluding He II 4686A emission) are essentially constant in line strength, but
are variable in velocity, establishing a double-lined binary orbit with P(orb)
= 1542d, e=0.45. We conduct a model-atmosphere analysis of the primary, and
find that the system is consistent with a O8: giant with a B1: main-sequence
secondary. Since the periodic 538-d changes are unrelated to orbital motion,
rotational modulation of a magnetically constrained plasma is strongly favoured
as the most likely underlying `clock'. An upper limit on the equatorial
rotation is consistent with this hypothesis, but is too weak to provide a
strong constraint.
