Superhumps and their Relation to the Disk Instability Model

Abstract: Since the discovery of superhumps in 1974, these photometric modulations have provided a crucial observational window into disk instabilities in cataclysmic variable stars, particularly the tidal instability associated with the 3:1 resonance. Over the past few decades, extensive time-resolved photometry has revealed a rich diversity of superhump-related phenomena, including delayed superhump development, early superhumps in WZ Sge-type dwarf novae, systematic stage A-B-C evolution, negative superhumps, and superhumps observed in related systems such as intermediate polars and AM CVn stars. In this invited review, we summarize key observational advances since the establishment of the thermal-tidal instability framework, discuss their theoretical interpretations within the disk instability model, and highlight remaining open problems. These developments have been driven by coordinated networks of amateur observers, wide-field robotic surveys, and continuous high-precision space-based photometry from Kepler and TESS. Together, they demonstrate that superhumps remain a powerful probe of disk dynamics, binary parameters, and the interplay between thermal, tidal, and geometric effects in accretion disks.