We present the discovery and characterization of HIP33609b, a transiting warm brown dwarf orbiting a late B-star, discovered by NASA's Transiting Exoplanet Survey Satellite as TOI-588b. HIP33609b is a large (Rb=1.580_-0.070_^+0.074^RJup) brown dwarf on a highly eccentric (e=0.560_-0.031_^+0.029^) orbit with a 39days period. The host star is a bright (V=7.3mag), Teff=10400_-660_^+800^K star with a mass of M=2.383_-0.095_^+0.10^M{sun} and radius of R=1.863_-0.082_^+0.087^R{sun}, making it the hottest transiting brown dwarf host star discovered to date. We obtained radial velocity measurements from the CHIRON spectrograph confirming the companion's mass of Mb=68.0_-7.1_^+7.4^MJup as well as the host star's rotation rate (vsini*=55.6{+/-}1.8km/s). We also present the discovery of a new comoving group of stars, designated as MELANGE-6, and determine that HIP33609 is a member. We use a combination of rotation periods and isochrone models fit to the cluster members to estimate an age of 150{+/-}25Myr. With a measured mass, radius, and age, HIP33609b becomes a benchmark for substellar evolutionary models.

Cone search capability for table J/AJ/165/268/fig6 (Rotation periods of candidate members of MELANGE-6 as a function of BP-RP color)