  Hot subdwarf B stars (sdBs) are extreme horizontal branch stars believed to
originate from close binary evolution. Indeed about half of the known sdB stars
are found in close binaries with periods ranging from a few hours to a few
days. The enormous mass loss required to remove the hydrogen envelope of the
red-giant progenitor almost entirely can be explained by common envelope
ejection. A rare subclass of these binaries are the eclipsing HW Vir binaries
where the sdB is orbited by a dwarf M star. Here we report the discovery of an
HW Vir system in the course of the MUCHFUSS project. A most likely substellar
object ($\simeq0.068\,M_{\rm \odot}$) was found to orbit the hot subdwarf
J08205+0008 with a period of 0.096 days. Since the eclipses are total, the
system parameters are very well constrained. J08205+0008 has the lowest
unambiguously measured companion mass yet found in a subdwarf B binary. This
implies that the most likely substellar companion has not only survived the
engulfment by the red-giant envelope, but also triggered its ejection and
enabled the sdB star to form. The system provides evidence that brown dwarfs
may indeed be able to significantly affect late stellar evolution.
