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abstract
As a part of our recent campaign to obtain complete, definitive, multiband light curves of very short period W UMa binary systems toward the short period limit (0.22d), we have obtained V,R,I CCD photemetry of BM Ursae Majoris. The observations were taken on 22 to 24 March 1991, inclusive, at Lowell Observatory, Flagstaff, Az. The 1.07 m Hall telescope was used in conjunction with the RCA CCD camera system. Four new precise epochs of minimum light were determined and improved linear and quadratic ephemerides were computed. The quadratic term -1.0(1)E-10 d, is statistically significant. In addition, the possibilities of discrete period changes and sinusoidal behavior are explored. The 1993 version of the Wilson code, complete with its two parameter bolometric limb darkening law, was used to analyze the light curves. The reflection effect was calculated to second order. The three-color simultaneous synthetic light curve solution is presented.
