Extrasolar Planets Occultation Research in Open Clusters (EXPLORE/OC) is a monitoring survey of eight Southern open clusters, designed to detect transits of close-in extrasolar giant planets. In total, the survey produced a sample of approximately 32000 stars with 2-10 mmag photometric precision (rms). For stars both in clusters and the Galactic field, we discriminate between planet transits and other sources that vary with amplitudes of a few percent by combining this excellent precision with high time-sampling (1000-2000 measurements per star, spread over three weeks).
The survey employs special techniques to measure and characterize the tens of thousands of sources. The data reduction pipeline incorporates neighbour subtraction and a generalized aperture photometry approach for mitigation of common observational systematic errors. We evaluate distances and spectral types for our sources by spectral energy distribution fitting. Armed with distance estimates, we are able to assign robust cluster membership probabilities to our sources.
In each cluster field, we find of order one transit-like variable, and dozens of eclipsing binary stars and pulsators. This number of planet candidates is in line with the expected frequency of planet occurrence derived from other planet searches.
Our eclipsing binary sample is large enough to offer a statistical view on the evolution of the fraction of eclipsing binaries in open clusters. Using our cluster contact binaries, we find evidence that the fraction of contact binaries increases on a timescale of Gyr, consistent with previous work. Extending those previous results to detached binaries, we find that detached binaries in clusters are destroyed on Gyr timescales. Pooling our detached binary detections from both clusters and the field, we find no support for the hypothesis that the binary mass ratio distribution is peaked towards equal masses.