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The eclipsing binary Theta1 Orionis B1, variable star designation BM Ori, isthe faintest of the four well-known Trapezium stars at the heart of the OrionNebula. The primary is a B3 star (~6Msun) but the nature of the secondary(~2Msun) has long been mysterious, since the duration and shape of primaryeclipse are inappropriate for ordinary stars. Here we report nearly continuousphotometric observations obtained with the MOST satellite over ~4 cycles of the6.47d binary period. The light curve is of unprecedented quality, revealing adeep, symmetric primary eclipse as well as a clear reflection effect andsecondary eclipse. In addition, there are other small disturbances, some ofwhich repeat at the same phase over the four cycles monitored. The shape of theprimary light curve has clearly evolved significantly over the past 40 yr.While its overall duration and depth have remained roughly constant, the slopesof the descent and ascent phases are significantly shallower now than in thepast and its distinctive flat-bottomed "pseudo-totality" is much less obviousor even absent in the most recent data. We further demonstrate that the primaryeclipse was detected at X-ray wavelengths during the Chandra COUP study. Thelight curve continues to be well modeled by a self-luminous and reflectivedisk-shaped object seen nearly edge-on orbiting the primary. The dramaticchange in shape over four decades is modeled as an opacity variation in atenuous outer envelope or disk of the secondary object. We presume that thesecondary is an extremely young protostar at an earlier evolutionary phase thancan be commonly observed elsewhere in the Galaxy and that the opacityvariations observed are related to its digestion of some accreted matter overthe last 50-100 yr. Indeed, this object deserves continued observational andtheoretical attention as the youngest known eclipsing binary system.

Dramatic Evolution of the Disk-Shaped Secondary in the Orion Trapezium Star Theta1 Ori B1 (BM Ori): MOST Satellite Observations