A Statistical Study of the O’Connell Effect in Eclipsing Binaries from the ASAS-SN Survey

The O’Connell effect is characterized by unequal maxima in the light curve of eclipsing binaries, while its physical origin remains unclear. The All-Sky Automated Survey for Supernovae provides more than a decade of all-sky monitoring data, including 154,530 eclipsing binaries, offering a valuable data set for large-scale investigations of this phenomenon. We identified a total of 749 targets exhibiting a significant O’Connell effect as the core sample, corresponding to an occurrence rate of 3.6%. This sample comprises 502 contact binaries, 217 semidetached binaries, and 30 detached binaries, with the size of the O’Connell effect as small as 0.02 mag. We compiled T eff from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, Gaia, and previous literature and found that the sample is mainly concentrated within orbital periods of 0.2–0.4 days and T eff of 4500–7500 K. Notably, EW-type binaries exhibit a clear bimodal structure in T eff distribution, with one group centered around 5500 K and another near 7000 K, possibly corresponding to the transition across the Kraft break. Our analysis indicates that the O’Connell effect manifests not only as a brightness asymmetry between the two maxima but also as a phase displacement of their occurrence. With increasing orbital period and T eff , the occurrence rate of the O’Connell effect decreases significantly, and nearly all cases exhibit positive O’Connell effects. Statistical evidence further suggests that the O’Connell effect is not a permanent property of binary systems but rather a dynamic phenomenon.