Abstract:
Differential BV photometry of DM UMa obtained over 19 nights and near simultaneous Hα spectroscopy obtained over 7 nights during the 1990-91 observing season are presented. From an analysis of the available data we find that the light curve of DM UMa shows both periods of small and large amplitudes with remarkable changes in its mean light level. Apparently, the amplitude of light variation does not depend either on the mean brightness of the system, or on its {DELTA}V_max_ or {DELTA}V_min_. The phases of light minima are found to lie on four well separated lines with different slopes. We find that the lifetime of a spot or spot group can be as short as two years. The modelling indicates the presence of spots at high latitudes. The Hα emission equivalent width in DM UMa shows a modulation with the photometric phase so that the maximum emission equivalent width is close to the minimum of the light curve. The spectrum of DM UMa obtained on 1991 Jan 7 shows strong evidence of a flare.
