Brown dwarf companions to G-type stars. I. Gliese 417b and Gliese 584C

We present astrometric and spectroscopic observations confirming that two n earby G dwarf systems (Gliese 417 = BD+36 degrees 2162 and Gliese 584AB =et a CrB AB) have a widely separated, L dwarf, substellar companion. Using age estimates of the G dwarf primaries, we estimate masses for these L dwarfs from theoretical evolutionary tracks. For the L4.5 dwarf Gl 417B we estimat e an age of 0.08-0.3 Gyr and a mass of 0.035 +/-0.015 M-circle dot. For the L8 dwarf Gl 584C we estimate an age of 1.0-2.5 Gyr and a mass of 0.060 +/- 0.015 M-circle dot. This latter object also shows evidence of spectrum vari ability, which may be due to surface inhomogeneities rotating into and out of view. These new companions are also compared to six other L dwarf and T dwarf companions previously recognized. For the L dwarf companions, ages im plied by the presence or absence of lithium are consistent with ages inferr ed from the primaries alone.