THE 71 SECOND OSCILLATION IN THE LIGHT-CURVE OF THE OLD NOVA DQ HERCULIS

DQ Her is an old nova (N1934), an eclipsing binary with an orbital per iod of 0.1936 days, and a prototype for cataclysmic variables containi ng rapidly rotating, accreting, magnetic white dwarfs. We have obtaine d high-speed, simultaneous U, B, V, and R photometry of DQ Her and hav e used the photometry to update and improve the ephemerides for the ec lipse and for the 71 s periodicity induced by the rotation of the whit e dwarf. We use the ephemerides to show that the present rate of mass transfer in DQ Her must be less than, but not much less than 3.4 x 10( -9) M. yr(-1) and that the magnetic moment of the white dwarf is near 2.7 x 10(32) G cm(3). The mean fractional semiamplitudes of the 71 s o scillation just before eclipse are Delta U/U = 0.010 +/- 0.001, Delta B/B = 0.012 +/- 0.001, Delta V/V = 0.011 +/- 0.001, and Delta R/R = 0. 009 +/- 0.001. The large change in oscillation amplitude with orbital phase seen in earlier data is no longer present. The 71 s oscillation undergoes a phase shift during eclipse, and we give a much improved me asurement of the shift. The phase increases smoothly by at least 100 d egrees during eclipse ingress, then jumps by 120 degrees at mid-eclips e, and finally climbs smoothly by another 140 degrees during eclipse e gress to end at +360 degrees. The morphology of the phase shift is not measurably different in the four passbands. The beamed radiation from the white dwarf in DQ Her irradiates its accretion disk, setting up a pattern of bright and dark regions that rotates with the white dwarf. We show that the pattern has two nearly equal bright regions in oppos ite directions on the disk and that the pattern rotates with a period of 142 s. The rotation period of the white dwarf is, thus, 142 s.