ANALYSIS OF CCD PHOTOMETRY FOR THE VERY SHORT-PERIOD ECLIPSING BINARYBM-URSAE-MAJORIS

CCD images of the fourteenth-magnitude variable BM Ursae Majoris were taken in the spring of 1991 at Lowell Observatory. An RCA CCD camera s ystem and a standard PMT were used. Four times of minimum Light were d etermined and improved linear and quadratic ephemerides were calculate d. A period study, spanning over a thirty-year interval, shows that th e system is undergoing a small but continuous period decrease of simil ar to 5x10(-8) d/yr. This may be due to angular momentum loss (AML) ca used by stellar winds. The VRI light curves formed from the present pr ecision observations, show that BM UMa is a W-type W UMa system. The f irst synthetic light curve solution of BM UMa is also presented. This solution reveals that BM UMa is a contact binary consisting of two ear ly-It spectral-type components with a fill out of similar to 20% and a mass ratio of similar to 0.5. A substantial temperature difference of similar to 400 K was determined. Both unspotted and spotted models we re calculated. The spotted model indicates the presence of a region of enhanced brightness in the neck of the secondary component. This may be attributed to fluid dynamics of mass in transit rather than to magn etic activity.