Superhumps in cataclysmic binaries. XIX. DV Ursae Majoris

DV Ursae Majoris is deeply eclipsing dwarf nova which shows very powerful s uperhumps when it attains superoutburst. We report detailed observations of the 1997 and 1999 eruptions. Some of the results reproduce what has been l earned from other eclipsing dwarf novae: that the disk becomes very large i n outburst, that superhumps develop in a few days, and that superhumps rema in strong even after the disk has shrunk by more than 30%. The mean superhu mp period was 0.08870(8) day, but in both eruptions the period decreased wi th (P) over dot approximate to -6 x 10(-5). Globally distributed coverage o f the 1997 eruption revealed two other interesting features: a transient st rong modulation at the orbital period at the peak of eruption, and intricat e fine structure in the harmonics of the main superhump signal. In particul ar, we found that the second harmonic occurred not at 3 omega -3 Omega as e xpected (where omega and Omega are, respectively, the orbital and "precessi on" frequencies), but at 3 omega -2 Omega and 3 omega-Omega. The strong orb ital modulation may have arisen from enhanced mass transfer from the second ary. We also report photometry at quiescence, which separates the luminous contr ibutions of the white dwarf, accretion disk, and secondary star. We estimat e a distance of 350 +/- 120 pc. Analysis of the eclipse suggests i = 84.deg rees0 +/- 0.degrees8, q = 0.155 +/- 0.015, M-2 = 0.14 +/- 0.02 M., M-1 = 0. 90 +/- 0.13 M..