2013A&A...559A..71G

The goal of our ongoing study of exoplanet host stars is to determine the true multiplicity rate of such stars. For this purpose we aim to research a statistically significant and homogeneously analyzed sample of stars around which planets have been found by the radial velocity or the transiting method. Ultimately, we want to determine if there are significant differences between planets in single- and multiple-star systems. This could indicate different formation scenarios. In this research note, we specifically investigate the companionship of three low-mass stellar candidate companions to exoplanet hosts via second epoch imaging and astrometry. We used various high-resolution imaging instruments in order to search for low-mass stellar companions to exoplanet host stars. Images were taken at a minimum of two observation epochs to check whether detected companion candidates are co-moving with the primary stars. We used the known photometric data of the primary stars together with theoretical evolutionary models in order to calculate detection limits for all targets and to estimate the mass of confirmed companions. With our astrometric data we can confirm for the first time that the exoplanet host star HAT-P-8 has a low-mass stellar companion with an absolute magnitude in the K-band of 7.37±0.15mag and an estimated mass of 0.25M_☉. However, in our high-resolution VLT/NACO observation the source appears elongated and could therefore be a close binary. If both of the binary components are equally massive, this binary could have a higher total mass of ∼0.35M_☉. In addition, we were able to calculate detection limits for the HAT-P-30 system and the HD44219 system and showed that companion candidates around these objects are most likely background objects.