NEAR SIMULTANEOUS BV-PHOTOMETRY AND H-ALPHA SPECTROSCOPY OF THE RS-CANUM-VENATICORUM BINARY DM-URSAE-MAJORIS

Differential BV photometry of DM UMa obtained over 19 nights and near simultaneous Halpha spectroscopy obtained over 7 nights during the 199 0-91 observing season are presented. From an analysis of the available data we find that the light curve of DM UMa shows both periods of sma ll and large amplitudes with remarkable changes in its mean light leve l. Apparently, the amplitude of light variation does not depend either on the mean brightness of the system, or on its DELTAV(max) or DELTAV (max). The phases of light minima are found to lie on four well separa ted lines with different slopes. We find that the lifetime of a spot o r spot group can be as short as two years. The modelling indicates the presence of spots at high latitudes. The Halpha emission equivalent w idth in DM UMa shows a modulation with the photometric phase so that t he maximum emission equivalent width is close to the minimum of the li ght curve. The spectrum of DM UMa obtained on 1991 Jan 7 shows strong evidence of a flare.