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The multiplicity of exoplanet host stars. Spectroscopic confirmation of the companions GJ 3021 B and

Aims. We present new results from our ongoing multiplicity study of exoplanet host stars and present a list of 29 confirmed planet host multiple-star systems. Furthermore, we discuss the properties of these stellar systems and compare the properties of exoplanets detected in these systems with those of planets orbiting single stars. Methods. We used direct imaging to search for wide stellar and substellar companions of exoplanet host stars. With infrared and/or optical spectroscopy, we determined the spectral properties of the newly-found co-moving companions. Results. We obtained infrared H -a ndK-band spectra of the co-moving companion GJ 3021 B. The infrared spectra and the apparent H-band photometry of the companion is consistent with an M3–M5 dwarf at the distance of the exoplanet host star. HD 40979 AB is a wide planet host stellar system, with a separation of ∼ 6400 AU. The companion to the exoplanet host star turned out to be a close stellar pair with a projected separation of ∼130 AU, hence, this system is a new member of those rare planet host triple-star systems of which only three other systems are presently known. HD 27442 AB is a wide binary system listed in the Washington Double Star Catalogue, whose common proper motion was recently confirmed. This system is composed of the subgiant HD 27442 A hosting the exoplanet, and its faint companion HD 27442 B. The visible and infrared J-, H-, and KS-band photometry of HD 27442 B at the distance of the primary star shows that the companion is probably a white dwarf. Our multi-epochs SofI imaging observations confirm this result and even refine the suggested physical characteristics of HD 27442 B. This companion should be a relatively young, hot white dwarf with an effective temperature of ∼14 400 K, and cooling age of ∼220 Myr. Finally, we could unambiguously confirm the white dwarf nature of HD 27442 B with follow-up optical and infrared spectroscopy. The spectra of the companion show Hydrogen absorption features of the Balmer, Paschen, and Bracket series. With its subgiant primary and the white-dwarf companion, the HD 27442 AB system is the most evolved planet host stellar system known today. The mass-period and eccentricity-period correlation of planets around single stars and those residing in multiple-star systems seem different for the short-period planets. The distribution functions of planet orbital elements (P, e) are identical, while the mass-distribution (m sin(i)) exhibits one difference. While both planet populations exhibit a peak in their mass-distribution at about 1 MJup, the frequency of more massive planets continually decreases in single-star systems, whereas the mass-distribution of planets residing in multiple-star systems exhibits a further peak at about 4 MJup. This indicates that the mass-distributions of the two planet populations might differ in the intermediate mass-range between 2 and 6 MJup.