Stellar companions to exoplanet host stars: Lucky Imaging of transiting planet hosts

Observed properties of stars and planets in binary/multiple star systems provide clues to planet formation and evolution. We extended our survey for visual stellar companions to the hosts of transiting exoplanets by 21 stars, using the Lucky Imaging technique with the two AstraLux instruments: AstraLux Norte at the Calar Alto 2.2-m telescope and AstraLux Sur at the European Southern Observatory 3.5-m New Technology Telescope at La Silla. Typically, a sensitivity to companions of magnitude difference Δz′ 4 is achieved at angular separation ρ = 0.5 arcsec and Δz′ 6 for ρ = 1 arcsec. We present observations of two previously unknown binary candidate companions, to the transiting planet host stars HAT-P-8 and WASP-12, and derive photometric and astrometric properties of the companion candidates. The common proper motions of the previously discovered companion candidates with the exoplanet host stars TrES-4 and WASP-2 are confirmed from follow-up observations. A Bayesian statistical analysis of 31 transiting exoplanet host stars observed with AstraLux suggests that the companion star fraction of planet hosts is not significantly different from that of solar-type field stars, but that the binary separation is on average larger for planet host stars.