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We present the results of near-infrared, follow-up imaging and spectroscopicobservations at VLT, aimed at characterizing the long-period companions of theexoplanet host stars HD196885, HD1237 and HD27442. The three companions werepreviously discovered in the course of our CFHT and VLT coronographic imagingsurvey dedicated to the search for faint companions of exoplanet host stars. Weused the NACO near-infrared adaptive optics instrument to obtain astrometricfollow-up observations of HD196885 A and B. The long-slit spectroscopic mode ofNACO and the integral field spectrograph SINFONI were used to carry out alow-resolution spectral characterization of the three companions HD196885 B,HD1237 B and HD27442 B between 1.4 and 2.5 microns. We can now confirm that thecompanion HD196885 B is comoving with its primary exoplanet host star, aspreviously shown for HD1237 B and HD27442 B. We find that both companionsHD196885 B and HD1237 B are low-mass stars of spectral type M1V and M4Vrespectively. HD196885 AB is one of the closer (~23 AU) resolved binaries knownto host an exoplanet. This system is then ideal for carrying out a combinedradial velocity and astrometric investigation of the possible impact of thebinary companion on the planetary system formation and evolution. Finally, weconfirm via spectroscopy that HD27442 B is a white dwarf companion, the thirdone to be discovered orbiting an exoplanet host star, following HD147513 andGliese 86. The detection of the broad Bracket gamma line of hydrogen indicatesa white dwarf atmosphere dominated by hydrogen.

Characterization of the long-period companions of the exoplanet host stars: HD196885, HD1237 and HD27442