PLANETS IN STELLAR CLUSTERS EXTENSIVE SEARCH. IV. A DETECTION OF A POSSIBLE TRANSITING PLANET CANDIDATE IN THE OPEN CLUSTER NGC 21581

We have undertaken a long-term project, Planets in Stellar Clusters Extensive Search, to search for transiting planets in open clusters. In this paper we present the results for NGC 2158, an intermediate-age, populous cluster. We have monitored the cluster for over 260 hours, spread over 59 nights. We have detected one candidate transiting low-luminosity object, with an eclipse depth of 3.7% in the R band. If the host star is a member of the cluster, the eclipse depth is consistent with a 1.7RJ object. Cluster membership of the host is supported by its location on the cluster main sequence and its close proximity to the cluster center (2ʹ). We have discovered two other stars exhibiting low-amplitude (4%–5%) transits, V64 and V70, but they are most likely blends or field stars. Given the photometric precision and temporal coverage of our observations and the current best estimates for the frequency and radii of short-period planets, the expected number of detectable transiting planets in our sample is 0.13. We have observed four outbursts for the candidate cataclysmic variable V57. We have discovered 40 new variable stars in the cluster, bringing the total number of identified variables to 97, and present for them high-precision light curves, spanning 13 months.