An intensive CCD photometry campaign to observe DW Ursae Majoris

We report on a coordinated observing campaign in April and May 2008 to study the eclipsing dwarf nova DW Ursae Majoris. This belongs to the group of SW Sex stars, nova-like variables containing accretion disks which exhibit superhumps in their light curves suggesting that their accretion disks are elliptical and precessing on time scales of a few days due to tidal interactions with the companion star. It has been suggested that the changing geometry will cause the depth of eclipses to be modulated on the accretion disk precession period. The aim of this campaign was to provide for the first time sufficient continuous photometric coverage of an eclipsing super-humper to test this hypothesis. 26 experienced amateur CCD photometrists in 7 countries participated in the project and altogether made almost 55,000 magnitude measurements over a 4 week period, keeping DW UMa under observation for more than 50% of the time. The results provide direct measurements of the orbital, super-hump and disk precession periods, confirming unambiguously that the superhump signal is a beat between the orbital and precession periods. They also reveal modulation not only of the eclipse depth but also of the eclipse time of minimum and width on the accretion disk precession period. The project is a good example of cooperation between the amateur and professional communities to address an open research issue