A Period and a Prediction for the Of?p Spectrum Alternator HD 191612

The observational picture of the enigmatic O-type spectrum variable HD191612has been sharpened substantially. A symmetrical, low-amplitude light curve witha period near 540 d has recently been reported from Hipparcos photometry. Thisperiod satisfies all of the spectroscopy since at least 1982, includingextensive new observations during 2003 and 2004, and it has predicted the nexttransition during September--October 2004. Measurements of the H alphaequivalent width reveal a sharp emission peak in the phase diagram, in contrastto the apparently sinusoidal light curve. The He II absorption-line strength isessentially constant, while He I varies strongly, possibly filled in byemission in the O6 state, thus producing the apparent spectral-type variations.The O8 state appears to be the "normal" one. Two intermediate O7 observationshave been obtained, which fall at the expected phases, but these are the onlymodern observations of the transitions so far. The period is too long forrotation or pulsation; although there is no direct evidence as yet for acompanion, a model in which tidally induced oscillations drive an enhanced windnear periastron of an eccentric orbit appears promising. Further observationsduring the now predictable transitions may provide a critical test. Ultravioletand X-ray observations during both states will likely also prove illuminating.Comment: 7 pages, 3 figures, 1 table; scheduled for the 2004 December 10 issue of ApJL, Vol. 617, No. 1. ApJ