Direct detection of the companion of chi 1 Orionis

We present an H-band image of the companion of chi ¹ Orionis taken with the Keck adaptive optic system and NIRC 2 camera equipped with a 300 mas-diameter coronographic mask. The direct detection of this companion star enables us to calculate dynamical masses using only Kepler's laws (M_A =1.01+/-0.13 M_sun, M_B =0.15 +/- 0.02 M_sun), and to study stellar evolutionary models at a wide spread of masses. The application of Baraffe et al. (\cite{Baraffe1998}) pre-main-sequence models implies an age of 70-130 Myrs. This is in conflict to the age of the primary, a confirmed member of the Ursa Major Cluster with a canonical age of 300 Myrs. As a consequence, either the models at low masses underestimate the age or the Ursa Major Cluster is considerably younger than assumed.