A search for transiting extrasolar planet candidates in the OGLE-II microlens database of the galactic plane

Context. In the late 1990s, the Optical Gravitational Lensing Experiment (OGLE) team conducted the second phase of their long-term monitoring programme, OGLE-II, which since has been superseded by OGLE-III. All the monitoring data of this second phase, which was primarily aimed at finding microlensing events, have recently been made public. Aims. Fields in the OGLE-II survey have typically been observed with a cadence of once per night, over a period of a few months per year. We investigated whether these radically differently sampled data can also be used to search for transiting extrasolar planets, in particular in the light of future projects such as PanSTARRS and SkyMapper, which will monitor large fields, but mostly not at a cadence typical for transit studies. Methods. We selected data for ∼15 700 stars with 13.0 < I < 16.0 in three OGLE-II fields towards the galactic disc in the constellation Carina, each with 500-600 epochs of I-band photometry. These light curves were first detrended using Sys-Rem, after which they were searched for low amplitude transits using the Box Least Squares algorithm. Results. The detrending algorithm significantly decreased the scatter in the light curves, from an average of 0.5% down to 0.2-0.3% for stars with I < 15. Several dozens of eclipsing binaries and low amplitude transits were found, of which 13 candidates exhibit transits with such depth and duration that they are possibly caused by an object with a radius less than twice that of Jupiter. Eleven out of these thirteen candidates show significant ellipsoidal light variations and are unlikely to host a transiting extrasolar planet. However, OGLE2-TR-L9 (CAR_SC2_75679), is an excellent planet candidate comparable to the known OGLE-III transiting planets, and deserves further follow-up observations.