A 16-yr photometric campaign on the eclipsing novalike variable DW Ursae Majoris

We present an analysis of photometric observations of the eclipsingnovalike variable DW UMa made by the CBA consortium between 1999 and 2015. Analysis of 372 new and 260 previously published eclipsetimings reveals a 13.6 year period or quasi-period in the times of minimum light. The seasonal light curves show a complex spectrum of periodic signals: both positive and negative superhumps, likely arising from a prograde apsidal precessionand a retrograde nodal precessionof the accretion disc. These signals appear most prominently and famously as sidebands of the orbital frequency but the precessionfrequencies themselves, at 0.40 and 0.22 cycles per day, are also seen directly in the power spectrum. The superhumps are sometimes seen together and sometimes separately. The depth, width and skew of eclipsesare all modulated in phase with both nodal and apsidal precessionof the tilted and eccentric accretion disc. The superhumps, or more correctly the precessional motions which produce them, may be essential to understanding the mysterious SW Sextantis syndrome. Disc wobble and eccentricity can both produce Doppler signatures inconsistent with the true dynamical motions in the binary, and disc wobble might boost the mass-transfer rate by enabling the hot white dwarfto directly irradiate the secondary star.