A low-mass stellar companion of the planet host star HD 75289

We report on the detection of a new low-mass stellar companion of HD 75289, a G0V star that harbors one known radial-velocity planet (Udry et al. \cite{Udry00}, A&A, 356, 590). Comparing an image from 2MASS with an image we obtained with SofI at the ESO 3.58 m NTT three years later, we detected a co-moving companion located 21.465±0.023 arcsec (621±10 AU at 29 pc) east of HD 75289. A second SofI image taken 10 months later confirmed the common proper motion of HD 75289 B with its host star. The infrared spectrum and colors of the companion are consistent with an M 2 to M 5 main-sequence star at the distance of HD 75289. No further (sub)stellar companion down to H=19 mag could be detected. With the SofI detection limit we can rule out additional stellar companions beyond 140 AU and substellar companions with masses m≥0.050 M⊙ from 400 AU up to 2000 AU. Based on observations obtained on La Silla in ESO programs 68.C-008 and 70.C-0116(A).