1995PASP..107..136S

CCD images of the fourteenth magnitude variable BM Ursae Majoris were taken in the spring of 1991. The observations were taken at Lowell Observatory. An RCA CCD camera system and a standard PMT were used. Four times of minimum light were determined and improved linear and quadratic ephemerides were calculated. A period study, spanning over a thirty year interval, shows that the system is undergoing a small but continuous period decrease of ∼5 X 10^-8 d/yr. This may be due to angular momentum loss (AML) caused by stellar winds. The V, R, I light curves formed from the present precision observations, show that BM UMa is a W-type W UMa system. The first synthetic light curve solution of BM UMa is also presented. This solution reveals that BM UMa is a contact binary consisting of two early K spectral type components with a fill out of ∼20% and a mass ratio of ∼0.5. A subtantial temperature difference of ∼400 K was determined. Both unspotted and spotted models were calculated. The spotted model indicates the presence of a region of enhanced birghtness in the neck of the secondary component. This may be attributed to fluid dynamics of mass in transit rather than to magnetic activity.