Time-resolved spectrophotometry of DQ Herculis - A wavelength-dependent phase shift in 71 second pulsations of He II lambda 4686

We have studied the spectral changes which occur in DQ Herculis during the 71 5 pulsation period. We find that the amplitude of the modulation is virtually independent of wavelength in the continuum from 3600 to 6100 A. However, the emission line He ii A4686 is more strongly modulated than the underlying continuum and exhibits an unexpected effect: the pulse phase increases rapidly with increasing wavelength across the line. This effect can be understood in terms of a simple model in which the pulsations arise at the inner edge of the accretion disk, excited by radiation which originates at hot spots on the white dwarf and which sweeps around the disk as the degenerate star rotates. A similar model in which the pulsations arise predominantly from the back half of the surface of the disk, although more difficult to make quantitative, appears in several respects to be more promising. The evident relation between the phase shift across the emission line and the broad-band photometric phase shift through eclipse, discovered by Warner et al., is also discussed. Subject headings: stars: accretion - stars: eclipsing binaries - stars: individual - stars: novae - stars: pulsation