Photometric study of the RS CVn binary DM Ursa Majoris

Abstract

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 fainter 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 assumes that large discrete spots are responsible for the observed light variation; the method of least squares was employed to derive the best-fit spot parameters. Observationally, we find that from 1984 onwards the brightness at light curve maximum has increased monotonically whereas the amplitude of light variation has remained within a narrow range 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 corresponding period. We derive a mean spot temperature of 3400 +/- 60 K from the data obtained during 10 of the observing seasons.