A Search for the O’Connell Effect in Contact Binaries in the Era of Big Sky Surveys. I.

Abstract Asymmetries between the two maxima of the light curves of contact binaries, known as the O’Connell effect, have been identified using the All Sky Automated Survey, All-Sky Automated Survey for Supernovae, Northern Sky Variability Survey, Optical Gravitational Lensing Experiment, and Optical Monitoring Camera sky surveys. The morphology of the mean light curve has been analyzed for a total of 43,659 objects already classified as contact binaries. After applying several filters and criteria, we present a sample of 797 systems representing the largest sample to date of maxima asymmetries of EW-type binaries with ∣Δ m ∣ up to 0.13 mag. We also added information on distance and temperature from Gaia DR3 and The Large Sky Area Multi-Object Fiber Spectroscopic Telescope. We investigated the statistical distributions of periods, temperatures, signs of maximum asymmetry, O’Connell effect ratio and light-curve asymmetry indices of asymmetry, and their correlations. While starspots could explain the different maxima asymmetries in stars with convection zones and differential rotations, the discovery of sizeable asymmetric maxima in 161 early-type systems above the upper limit of the Kraft break ( T > 6700 K) may indicate a different mechanism. We also report systems with peculiar unequal maxima.