2005A&A...438.1123P

We present the results of our high-resolution spectroscopic follow-up of 42 planetary transit candidates in Carina from the OGLE survey. This follow-up has already allowed the discovery of three new transiting exoplanets, OGLE-TR-111, 113 and 132. Here we analyse the data for the remaining 39 candidates. The radial velocity data show that most of them are eclipsing binaries, in very varied configurations. Precise radial velocity orbits were derived for 15 binaries, revealing 9 transits of small stars (generally M-dwarfs) in front of F-G dwarfs, 1 grazing equal-mass eclipsing binary, 4 triple and 1 quadruple systems. A further 14 systems appear binary, but the exact orbit is uncertain or was not determined. 2 objects do not show any radial velocity variations in phase with the transit signal, and 6 do not possess spectral lines strong enough for a reliable cross-correlation function to be measured. Among these last two categories, up to 6 objects are suspected false positives of the photometric transit detection. 2 objects are unsolved cases that deserve further observations. Our study illustrates the wide variety of cases that can mimic photometric planetary transits, and the importance of spectroscopic follow-up. Multi-fiber capacities and an optimized follow-up strategy, which we present here, can help deal with the high number of candidates that are likely to be found in the near future. An important by-product of this study is the determination of exact masses and radii for five very low-mass stars, including two at the very edge of the stellar domain, OGLE-TR-106 (M=0.116±0.021M_☉) and OGLE-TR-122 (M=0.092±0.009M_☉). The radius of these objects is consistent with theoretical expectations. Two further objects, OGLE-TR-123 and OGLE-TR-129, may harbour transiting companions near the brown-dwarf/stellar limit (M≃0.07M_☉), whose confirmation requires further high-resolution spectroscopic monitoring. No transiting massive planets (M=2-10M_J) were detected, confirming the rarity of such systems at short period as indicated by Doppler surveys. No light (M<0.5M_J), large (R>R_J) planets were found either, indicating that ``hot Saturns'' generally have smaller radii than hot Jupiters. Three short period binaries with a M-dwarf companion show definite orbital eccentricities, with periods ranging from 5.3 to 9.2 days. This confirms theoretical indications that orbital circularisation in close binaries is less efficient for smaller companion masses. We also discuss the implications of our results for the statistical interpretation of the OGLE planetary transit survey in Carina in terms of planet frequency and detection efficiency. We find that the actual transit detection threshold is considerably higher than expected from simple estimates, and very strongly favours the detection of planets with periods shorter than about 2 days. The apparent contradition between the results of the OGLE transit survey and Doppler surveys can be resolved when this detection bias is taken into account.