Detection and characterisation of stellar companions to transiting extrasolar planets through high resolution imaging

Since the discovery over two decades ago of the first `hot Jupiter', there has been persistent debate regarding the formation and origins of this class of planets. Some of the many `migration' theories for hot Jupiters suggest that these planets form in binary star systems, being driven inwards towards their host star by dynamical interactions. If this is the case, many hot Jupiters should be found in multiple stellar systems, which might only be resolved with dedicated high resolution imaging. An additional use for high resolution imaging is the detection of previously unresolved sources of contaminating light, which can significantly bias the properties derived for planets. Observations were carried out of 156 transiting exoplanet systems with lucky imaging, and of 46 systems with near infrared adaptive options, in order to detect stars at small angular separation, and to identify binary companions. The lucky imaging survey identified 39 plausible multiple systems, and historical measurements allowed binary orbits to be determined for WASP-77 and WASP-85. The AO survey identified companion stars to 26 of its 46 targets, although further work is needed to confirm the nature of several of these companions. A surprising trend among the confirmed binary companions, both in this work and others, is that they are generally much less massive than the planet host stars. This contrasts with studies of solar type stars have shown that binaries most often consist of components with similar masses, and further investigation is needed to explain the trend among exoplanet host stars. The Gaia survey has proved a useful source of additional astrometric data in this work, and is already competitive with lucky imaging for detection of companions. Future Gaia data releases will further improve this further, and detailed searches for binary companions present in the Gaia data are strongly encouraged