A Spiral Structure in the Disk of EX Draconis on the Rise to Outburst Maximum

We report on the R-band eclipse mapping analysis of high-speed photometry of the dwarf nova EX Dra on the rise to the maximum of the 1995 November outburst. The eclipse map shows a one-armed spiral structure of ~180° in azimuth, extending in radius from R ≃ 0.2RL1 to 0.43RL1 (where RL1 is the distance from the disk center to the inner Lagrangian point), that contributes about 22% of the total flux of the eclipse map. The spiral structure is stationary in a reference frame corotating with the binary and is stable for a timescale of at least five binary orbits. The comparison of the eclipse maps on the rise and in quiescence suggests that the outbursts of EX Dra may be driven by episodes of enhanced mass transfer from the secondary star. Possible explanations for the nature of the spiral structure are discussed.