BM Orionis, the Eclipsing Binary in the Trapezium

Photoelectric UBV observations of BM Orionis were obtained by offsetting in a consistent manner which corrected for both nebulosity and scattered light. A complete li ht curve was obtained in V, with U and B observations made at maximum an minimum. A secondary eclipse of 0" O4 was found; rimary eclipse, shown to be an occultation, had a totali lastin nine hours. T e new ephemeris is JD 2440265.343 + 6d470525 E. So ution o the V light curve gave k = 0.29 I 0.03, rg = O.274,Lg = 0.591, audi = 83 & Thesha ekandde thk agreed very well. The V magnitudes and the B- V and U- B in ices were 93, , and - 0'!'58 for the hotter star and , , and - for the cooler star, respectively. A plot on the color-color diagram indicated that the hotter star is B2 or B3, in good agreement with spectral classifications. The cooler star is anomalous and can be considered an Al star with U-B = Withft9)') = 0.35 9)' a, from Struve and Titus and with the 9)'t:L relation applied only to the B star, the resulting dimensions are 5.4 9)'u,2.5 Ro., Mv = 07 for the B star and 2.89)'to., 8.5 R Mv - for the A star. The B star is thus below the observational ZAMS of Blaauw but on the theoretical ZAMS of Iben; the A star is two magnitudes overluminous for its mass. The dimensions imply that the B star is about 5 X 10 years old, seemingly older than the Trapezium (10 years) presumably because of its binary nature. With = I and m - M = 8"'2 I 0"'2 for the Trapezium, the resulting Av/ 4 6 0 8 implying that this ratio is larger than 3.0 but not as large as 5.5.