Accretion Disc Evolution in DW Ursae Majoris: A Photometric Study

We present an analysis of CCD photometric observations of the eclipsing novalike cataclysmic variable DW UMa obtained in two different luminosity states: high and intermediate. The star presents eclipses with very different depth: ∼ 1.2 mag in the high and ∼ 3.4 mag in the intermediate state. Eclipse mapping reveals that this difference is almost entirely due to the changes in the accretion disc radius: from ∼ 0.5RL1 in the intermediate state to ∼ 0.75RL1 in the high state (RL1 is the distance from the white dwarf to the first Lagrangian point). In the intermediate state, the entire disc is eclipsed while in the high state, its outer part remains visible. We also find that the central intensity of the disc is nearly the same in the two luminosity states and that it is the increase of the disc radius that is responsible for the final rise from the 1999/2000 low state. We find that the intensity profile of the disc is rather flat and suggest a possible explanation. We also discuss the effect of using a more realistic limb-darkening law on the disc temperatures inferred from eclipse mapping experiments. Periodogram analysis of the high state data reveals ”positive superhumps” with a period of 0. d1455 in 2002 and 0. d1461 in 2003, in accord with the results of Patterson et al. However, we cannot confirm the quasi-periodic oscillations reported by these authors. We obtain an updated orbital ephemeris of DW UMa: Tmin[HJD] = 2446229.00687(9) + 0. d136606527(3)E.