Context. GJ 1214b, the 6.55 Earth-mass transiting planet recently discovered by the MEarth team, has a mean density of ∼35% of that of the Earth. It is thought that this planet is either a mini-Neptune, consisting of a rocky core with a thick, hydrogen-rich atmosphere, or a planet with a composition dominated by water. Aims. In the case of a hydrogen-rich atmosphere, molecular absorption and scattering processes may result in detectable radius variations as a function of wavelength. The aim of this paper is to measure these variations. Methods. We have obtained observations of the transit of GJ 1214b in the r-and I-band with the Isaac Newton Telescope (INT), in the g-, r-, i-and z-bands with the 2.2 m MPI/ESO telescope, in the K s -band with the Nordic Optical Telescope (NOT), and in the K c -band with the William Herschel Telescope (WHT). By comparing the transit depth between the the different bands, which is a measure for the planet-to-star size ratio, the atmosphere is investigated. Results. We do not detect clearly significant variations in the planet-to-star size ratio as function of wavelength. Although the ratio at the shortest measured wavelength, in g-band, is 2σ larger than in the other bands. The uncertainties in the K s and K c bands are large, due to systematic features in the light curves. Conclusions. The tentative increase in the planet-to-star size ratio at the shortest wavelength could be a sign of an increase in the effective planet-size due to Rayleigh scattering, which would require GJ 1214b to have a hydrogen-rich atmosphere. If true, then the atmosphere has to have both clouds, to suppress planet-size variations at red optical wavelengths, as well as a sub-solar metallicity, to suppress strong molecular features in the near-and mid-infrared. However, star spots, which are known to be present on the hoststar's surface, can (partly) cancel out the expected variations in planet-to-star size ratio, because the lower surface temperature of the spots causes the effective size of the star to vary with wavelength. A hypothetical spot-fraction of ∼10%, corresponding to an average stellar dimming of ∼5% in the i-band, would be able to raise the near-and mid-infrared points sufficiently with respect to the optical measurements to be inconsistent with a water-dominated atmosphere. Modulation of the spot fraction due to the stellar rotation would in such case cause the observed flux variations of GJ 1214.

We report on the discovery of four ultra‐short‐period (P ≤ 0.18 d) eclipsing M‐dwarf binaries in the Wide‐Field Camera (WFCAM) Transit Survey. Their orbital periods are significantly shorter than that of any other known main‐sequence binary system, and are all significantly below the sharp period cut‐off at P ∼ 0.22  d as seen in binaries of earlier‐type stars. The shortest‐period binary consists of two M4‐type stars in a P = 0.112  d orbit. The binaries are discovered as part of an extensive search for short‐period eclipsing systems in over 260 000 stellar light curves, including over 10 000 M‐dwarfs down to J = 18 mag, yielding 25 binaries with P ≤ 0.23 d. In a popular paradigm, the evolution of short‐period binaries of cool main‐sequence stars is driven by the loss of angular momentum through magnetized winds. In this scheme, the observed P ∼ 0.22 d period cut‐off is explained as being due to time‐scales that are too long for lower‐mass binaries to decay into tighter orbits. Our discovery of low‐mass binaries with significantly shorter orbits implies that either these time‐scales have been overestimated for M‐dwarfs, e.g. due to a higher effective magnetic activity, or the mechanism for forming these tight M‐dwarf binaries is different from that of earlier‐type main‐sequence stars.

We report the discovery of WTS-2 b, an unusually close-in 1.02-day hot Jupiter (M P = 1.12M J , R P = 1.363R J ) orbiting a K2V star, which has a possible gravitationally-bound M-dwarf companion at 0.6 arcsec separation contributing ∼ 20 percent of the total flux in the observed J-band light curve. The planet is only 1.5 times the separation from its host star at which it would be destroyed by Roche lobe overflow, and has a predicted remaining lifetime of just ∼ 40 Myr, assuming a tidal dissipation quality factor of Q ′ ⋆ = 10 6 . Q ′ ⋆ is a key factor in determining how frictional processes within a host star affect the orbital evolution of its companion giant planets, but it is currently poorly constrained by observations. We calculate that the orbital decay of WTS-2 b would correspond to a shift in its transit arrival time of T shift ∼ 17 seconds after 15 years assuming Q ′ ⋆ = 10 6 . A shift less than this would place a direct observational constraint on the lower limit of Q ′ ⋆ in this system. We also report a correction to the previously published expected T shift for WASP-18 b, finding that T shift = 356 seconds after 10 years for Q ′ ⋆ = 10 6 , which is much larger than the estimated 28 seconds quoted in WASP-18 b discovery paper. We attempted to constrain Q ′ ⋆ via a study of the entire population of known transiting hot Jupiters, but our results were inconclusive, requiring a more detailed treatment of transit survey sensitivities at long periods. We conclude that the most informative and straight-forward constraints on Q ′ ⋆ will be obtained by direct observational measurements of the shift in transit arrival times in individual hot Jupiter systems. We show that this is achievable across the mass spectrum of exoplanet host stars within a decade, and will directly probe the effects of stellar interior structure on tidal dissipation.

Context. Only recently it has become possible to measure the thermal emission from hot-Jupiters at near-Infrared wavelengths using ground-based telescopes, by secondary eclipse observations. This allows the planet flux to be probed around the peak of its spectral energy distribution, which is vital for the understanding of its energy budget. Aims. The aim of the reported work is to measure the eclipse depth of the planet HAT-P-1b at 2.2 μm. This planet is an interesting case, since the amount of stellar irradiation it receives falls in between that of the two best studied systems (HD 209458 and HD 189733), and it has been suggested to have a weak thermal inversion layer. Methods. We have used the LIRIS instrument on the William Herschel Telescope (WHT) to observe the secondary eclipse of HAT-P-1b in the K s -band, as part of our Ground-based secondary eclipse (GROUSE) project. The observations were done in staring mode, while significantly defocusing the telescope to avoid saturation on the K = 8.4 star. With an average cadence of 2.5 s, we collected 6520 frames during one night. Results. The eclipse is detected at the 4-σ level, the measured depth being 0.109 ± 0.025%. The uncertainties are dominated by residual systematic effects, as estimated from different reduction/analysis procedures. The measured depth corresponds to a brightness temperature of 2136 +150 −170 K. This brightness temperature is significantly higher than those derived from longer wavelengths, making it difficult to fit all available data points with a plausible atmospheric model. However, it may be that we underestimate the true uncertainties of our measurements, since it is notoriously difficult to assign precise statistical significance to a result when systematic effects are important.

Star formation theory predicts that short-period M-dwarf binaries with highly unequal-mass components are rare. Firstly, the mass ratio of close binary systems is driven to unity due to the secondary preferentially accreting gas with high angular momentum. Secondly, both dynamical decay of multiple systems and interactions with tertiary stars that tighten the binary orbit will eject the lowest mass member. Generally, only the two most massive stars are paired after such interactions, and the frequency of tight unequal-mass binaries is expected to decrease steeply with primary mass. In this paper we present the discovery of a highly unequal-mass eclipsing Mdwarf binary, providing a unique constraint on binary star formation theory and on evolutionary models for low-mass binary stars. The binary is discovered using highprecision infrared light curves from the WFCAM Transit Survey (WTS) and has an orbital period of 2.44 d. We find stellar masses of M 1 = 0.53 ± 0.02 M ⊙ and M 2 = 0.143 ± 0.006 M ⊙ (mass ratio 0.27), and radii of R 1 = 0.51 ± 0.01 R ⊙ and R 2 = 0.174 ± 0.006 R ⊙ . This puts the companion in a very sparsely sampled and important late M-dwarf mass-regime. Since both stars share the same age and metallicity and straddle the theoretical boundary between fully and partially convective stellar interiors, a comparison can be made to model predictions over a large range of M-dwarf masses using the same model isochrone. Both stars appear to have a slightly inflated radius compared to 1 Gyr model predictions for their masses, but future work is needed to properly account for the effects of star spots on the light curve solution. A significant, subsynchronous, ∼2.56 d signal with ∼2% peak-to-peak amplitude is detected in the WFCAM light curve, which we attribute to rotational modulation of cool star spots. We propose that the subsynchronous rotation is either due to a stable star-spot complex at high latitude on the (magnetically active) primary (i.e. differential rotation), or to additional magnetic braking, or to interaction of the binary with a third body or circumbinary disk during its pre-main-sequence phase.

We present an analysis of the photometric variability of M dwarfs in the Wide Field Camera (WFCAM) Transit Survey. Although periodic light-curve variability in low mass stars is generally dominated by photospheric star spot activity, M dwarf variability in the J band has not been as thoroughly investigated as at visible wavelengths. Spectral type estimates for a sample of over 200 000 objects are made using spectral type-colour relations, and over 9600 dwarfs (J <17) with spectral types later than K7 were found. The light curves of the late-type sample are searched for periodicity using a Lomb-Scargle periodogram analysis. A total of 68 periodic variable M dwarfs are found in the sample with periods ranging from 0.16 to 90.33 d, with amplitudes in the range of similar to 0.009 to similar to 0.115 in the J band. We simulate active M dwarfs with a range of latitude-independent spot coverages and estimate a periodically variable fraction of 1-3 per cent for stars where spots cover more than 10 per cent of the star's surface. Our simulated spot distributions indicate that operating in the J band, where spot contrast ratios are minimized, enables variability in only the most active of stars to be detected. These findings affirm the benefits of using the J band for planetary transit searches compared to visible bands. We also serendipitously find a Delta J > 0.2 mag flaring event from an M4V star in our sample

Context. One of the biggest challenges facing large transit surveys is the elimination of false-positives from the vast number of transit candidates. A large amount of expensive follow-up time is spent on verifying the nature of these systems. Aims. We investigate to what extent information from the lightcurves can identify blend scenarios and eliminate them as planet candidates, to significantly decrease the amount of follow-up observing time required to identify the true exoplanet systems. Methods. If a lightcurve has a sufficiently high signal-to-noise ratio, a distinction can be made between the lightcurve of a stellar binary blended with a third star and the lightcurve of a transiting exoplanet system. We first simulate lightcurves of stellar blends and transiting planet systems to determine what signal-to-noise level is required to make the distinction between blended and non-blended systems as function of transit depth and impact parameter. Subsequently we test our method on real data from the first IRa01 field observed by the CoRoT satellite, concentrating on the 51 candidates already identified by the CoRoT team. Results. Our simulations show that blend scenarios can be constrained for transiting systems at low impact parameters. At high impact parameter, blended and non-blended systems are indistinguishable from each other because they both produce V-shaped transits. About 70% of the planet candidates in the CoRoT IRa01 field are best fit with an impact parameter of b > 0.85, while less than 15% are expected in this range considering random orbital inclinations. By applying a cut at b < 0.85, meaning that ∼15% of the potential planet population would be missed, the candidate sample decreases from 41 to 11. The lightcurves of 6 of those are best fit with such low host star densities that the planet-to-star size ratii imply unrealistic planet radii of R > 2 R Jup . Two of the five remaining systems, CoRoT1b and CoRoT4b, have been identified as planets by the CoRoT team, for which the lightcurves alone rule out blended light at 14% (2σ) and 31% (2σ). One system possesses a M-dwarf secondary, one a candidate Neptune. Conclusions. We show that in the first CoRoT field, IRa01, 85% of the planet candidates can be rejected from the lightcurves alone, if a cut in impact parameter of b < 0.85 is applied, at the cost of a <15% loss in planet yield. We propose to use this method on the Kepler database to study the fraction of real planets and to potentially increase the efficiency of follow-up.

The Wide Field Camera Transit Survey is a pioneer program aiming at for searching extra-solar planets in the near-infrared. The images from the survey are processed by a data reduction pipeline, which uses aperture photometry to construct the light curves. We produce an alternative set of light curves using the difference-imaging method for the most complete field in the survey and carry out a quantitative comparison between the photometric precision achieved with both methods. The results show that differencephotometry light curves present an important improvement for stars with J > 16. We report an implementation on the box-fitting transit detection algorithm, which performs a trapezoid-fit to the folded light curve, providing more accurate results than the boxfitting model. We describe and optimize a set of selection criteria to search for transit candidates, including the V-shape parameter calculated by our detection algorithm. The optimized selection criteria are applied to the aperture photometry and difference-imaging light curves, resulting in the automatic detection of the best 200 transit candidates from a sample of ∼475 000 sources. We carry out a detailed analysis in the 18 best detections and classify them as transiting planet and eclipsing binary candidates. We present one planet candidate orbiting a late G-type star. No planet candidate around M-stars has been found, confirming the null detection hypothesis and upper limits on the occurrence rate of short-period giant planets around M-dwarfs presented in a prior study. We extend the search for transiting planets to stars with J ≤ 18, which enables us to set a stricter upper limit of 1.1%. Furthermore, we present the detection of five faint extremely-short period eclipsing binaries and three M-dwarf/M-dwarf binary candidates. The detections demonstrate the benefits of using the difference-imaging light curves, especially when going to fainter magnitudes.