PHOTOMETRIC-STUDY OF THE RS-CVN BINARY DM URSA-MAJORIS

Differential BV photometry of DM UMa obtained on 77 nights during the six observing seasons over the years 1984-90 is presented. The phases of the light minima observed during 1988-90 are found to be nearly the same. The variation in B - V indicates that the star is redder at fai nter visual magnitudes. We have analyzed all the B and V light curves of DM UMa available from 1979 onwards by means of a spot model which a ssumes that large discrete spots are responsible for the observed ligh t variation; the method of least squares was employed to derived the b est-fit spot parameters. Observationally, we find that from 1984 onwar ds the brightness at light curve maximum has increased monotonically ( by approximately 0.20 mag), whereas the amplitude of light variation h as remained within a narrow range (0.16-0.23 mag) without any apparent trend; the modelling indicates a slow migration of the spots towards the equator, and a gradual decrease of the spot area during the corres ponding period. We derive a mean spot temperature of 3400 +/- 60 K fro m the data obtained during ten of the observing seasons.