Abstract. Ground based radial velocity (RV) searches continue to discover exoplanets below Neptune mass down to Earth mass. Furthermore, ground based transit searches now reach milli-mag photometric precision and can discover Neptune size planets around bright stars. These searches will find exoplanets around bright stars anywhere on the sky, their discoveries representing prime science targets for further study due to the proximity and brightness of their host stars. A mission for transit follow-up measurements of these prime targets is currently lacking. The first ESA S-class mission CHEOPS (CHaracterizing ExoPlanet Satellite) will fill this gap. It will perform ultra-high precision photometric monitoring of selected bright target stars almost anywhere on the sky with sufficient precision to detect Earth sized transits. It will be able to detect transits of RV-planets by photometric monitoring if the geometric configuration results in a transit. For Hot Neptunes discovered from the ground, CHEOPS will be able to improve the transit light curve so that the radius can be determined precisely. Because of the host stars' brightness, high precision RV measurements will be possible for all targets. All planets observed in transit by CHEOPS will be validated and their masses will be known. This will provide valuable data for constraining the mass-radius relation of exoplanets, especially in the Neptune-mass regime. During the planned 3.5 year mission, about 500 targets will be observed. There will be 20% of open time available for the community to develop new science programmes. a

Abstract.Our new algorithm for differential photometry solves the problem of identifying proper comparison stars without a prior detailed study of the field of view. The comparison stars' variability is determined in a self-consistent way, and their weighted average is used as a reference level. The maximum error in differential photometry using objects and reference stars of different spectral types is estimated. The results from these calculations show that the photometric band chosen greatly determines the level of accuracy achieved. Finally, an important application of high-precision differential photometry are planetary transits.

Aims. We wanted to determine the rotation parameters of GQ Lup A, thereby constraining the evolutionary history of the GQ Lup system. Methods. We have undertaken a photometric monitoring campaign on GQ Lup A consisting of two epochs spaced one year apart. We also searched the photometric archives to enlarge the data set. Results. We were able to determine the photometric period (8.45 ± 0.2 days) in both epochs in several photometric bands. This periodicity could also be found in some of the archival data. The combined false-alarm probability is 0.015. The variation is most likely caused by hot spots on the surface of GQ Lup A. This, combined with high-resolution spectra (v sin i) allows calculation of GQ Lup A's inclination (i = 27 ± 5• ). Radial velocity data also contains this period but is inconclusive. Nevertheless, the RV data supports the interpretation that hot spots cause the photometric variation. We use the known K-band variability, amplitude, and phase of GQ Lup A together with a new image of GQ Lup A+b, taken quasi-simultaneously with our monitoring of the star, to confirm the magnitude and, hence, luminosity of the companion.

We have carried out a search for co-moving stellar and substellar companions around 18 exoplanet host stars with the infrared camera MAGIC at the 2.2 m Calar Alto telescope, by comparing our images with images from the all sky surveys 2MASS, POSS I and II. Four stars of the sample namely HD 80606, 55 Cnc, HD 46375 and BD−10• 3166, are listed as binaries in the Washington Visual Double Star Catalogue (WDS). The binary nature of HD 80606, 55 Cnc, and HD 46375 is confirmed with both astrometry as well as photometry, thereby the proper motion of the companion of HD 46375 was determined here for the first time. We derived the companion masses as well as the longterm stability regions for additional companions in these three binary systems. We can rule out further stellar companions around all stars in the sample with projected separations between 270 AU and 2500 AU, being sensitive to substellar companions with masses down to ∼ 60 M Jup (S/N = 3). Furthermore we present evidence that the two components of the WDS binary BD−10• 3166 are unrelated stars, i.e this system is a visual pair. The spectrophotometric distance of the primary (a K0 dwarf) is ∼ 67 pc, whereas the presumable secondary BD−10• 3166 B (a M4 to M5 dwarf) is located at a distance of 13 pc in the foreground.