A second substellar companion in the Gliese 86 system - A brown dwarf in an extrasolar planetary system

We report observations using the ESO adaptive optics system ADONIS of the k nown extrasolar planetary system Gliese 86. This star has a known 4 M-Jup s in i planet in a 15.8 day orbit and exhibits an addi tional, large, long-pe riod, radial velocity drift (Queloz ct al. 2000). The coronographic images reveal a faint (J = 14.7, H = 14.4, K = 13.7) object at a projected distanc e of r = 1." 72 +/- 0." 02 and PA = 119 +/- 1 degrees. Gliese 86 and the di scovered object share the same proper motion, as confirmed by independent m easurements at three different epochs indicating that this system is gravit ationally bound. From the infrared colors and magnitudes we infer an approx imate spectral type for Gliese 86B at the transition from L to T dwarfs, al so called "early T dwarf" assuming the classification by Leggett et al. (20 00). Although present brown dwarf evolutionary models do not cover the mass and age range probed by this objects, an upper limit of the mass of about M-GJ86B less than or equal to 70 M-Jup can be inferred from the models by B araffe et al. (1998). Dusty model atmospheres appear not to be compatible w ith the IR colors.