  We present an H-band image of the companion of Chi 1 Orionis taken with the
Keck adaptive optic system and NIRC2 camera equipped with a 300mas-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. (1998) 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.
