Hot subdwarf B stars ( sdBs ) are core helium-burning stars located on the extreme horizontal branch .
About half of the known sdB stars are found in close binaries .
Their short orbital periods of 1.2 h to a few days suggest that they are post common-envelope systems .
Eclipsing hot subdwarf binaries are rare , but important to determine the fundamental stellar parameters .
Low-mass companions are identified by the reflection effect .
In most cases the companion is a main sequence star near the stellar mass limit . Here we report the discovery of an eclipsing hot subdwarf binary SDSS J162256.66+473051.1 ( J1622 ) of very short orbital period ( 0.0697 d ) , found in the course of the MUCHFUSS project .
The lightcurve shows grazing eclipses and a prominent reflection effect .
An analysis of the light- and radial velocity ( RV ) curves indicated a mass ratio of q = 0.1325 , an RV semiamplitude K = 47.2 km s ^ { -1 } , and an inclination of i = 72.33 ^ { \circ } .
We show that a companion mass of 0.064 M _ { \odot } , well below the hydrogen-burning limit , is the most plausible solution , which implies a mass close to the canonical mass ( 0.47 M _ { \odot } ) of the sdB star .
Therefore , the companion is a brown dwarf , which not only survived the engulfment by the red-giant envelope , but also triggered its ejection , and enabled the sdB star to form . The rotation of J1622 is expected to be tidally locked to the orbit .
However , J1622 rotates too slowly ( v _ { rot } = 74.5 \pm 7 km s ^ { -1 } ) to be synchronized , challenging tidal interaction models .