The long-period Algol-type binary KU Cygni (P=34.839d) displays double-lobed H emission lines at all orbital phases, indicating the existence of an accretion disk. The orbital separation of the components is adequately large, so that the mass-transfer stream does not impact the primary star, but rather loops around it to form a stable accretion disk. An analysis of time-resolved spectroscopic observations is presented within the basic model of an accretion disk. The estimated effective radius of the disk, based on eclipse timings of the V and R emission lobes, is 22 .
The mathematical technique of Fourier-filtered back-projection is applied to the one-dimensional spectra of KU Cyg to reconstruct a two-dimensional velocity map. This map, called a Doppler tomogram, displays the intensity distribution of H emission regions in velocity space. Restricted three-body calculations were used to determine the trajectory of the mass-transfer stream as it exits the L1 point at sonic velocities. The Doppler tomogram of KU Cyg reveals the existence of a symmetric accretion disk that extends out to the critical Roche lobe of the primary star. On the trailing side of the disk, an area of enhanced emission is located near the velocity trajectory of the stream, revealing the impact location of the mass-transfer stream with the outer edge of the accretion disk. The Keplerian velocity components of the enhanced emission estimate the impact location at 12.5 from the primary. There is no apparent evidence of emission along the trajectory of the mass-transfer stream. Currently, KU Cygni is only the second long-period Algol-type binary to be imaged with Doppler tomography.