2007A&A...475..723C

We present the results of near-infrared, follow-up imaging and spectroscopic observations at VLT, aimed at characterizing the long-period companions of the exoplanet host stars HD196885, HD1237 and HD27442. The three companions were previously discovered in the course of our CFHT and VLT coronographic imaging survey dedicated to the search for faint companions of exoplanet host stars. We used the NACO near-infrared adaptive optics instrument to obtain astrometric follow-up observations of HD196885A and B. The long-slit spectroscopic mode of NACO and the integral field spectrograph SINFONI were used to carry out a low-resolution spectral characterization of the three companions HD196885B, HD1237B and HD27442B between 1.4 and 2.5µm. We can now confirm that the companion HD196885B is comoving with its primary exoplanet host star, as previously shown for HD1237B and HD27442B. We find that both companions HD196885B and HD1237B are low-mass stars of spectral type M1±1V and M4±1V respectively. HD196885AB is one of the closer (∼23AU) resolved binaries known to host an exoplanet. This system is then ideal for carrying out a combined radial velocity and astrometric investigation of the possible impact of the binary companion on the planetary system formation and evolution. Finally, we confirm via spectroscopy that HD27442B is a white dwarf companion, the third one to be discovered orbiting an exoplanet host star, following HD147513 and Gliese 86. The detection of the broad Brγ line of hydrogen indicates a white dwarf atmosphere dominated by hydrogen.