  Hot subdwarf B stars (sdBs) in close binary systems are assumed to be formed
via common envelope ejection. According to theoretical models, the amount of
energy and angular momentum deposited in the common envelope scales with the
mass of the companion. That low mass companions near or below the core
hydrogen-burning limit are able to trigger the ejection of this envelope is
well known. The currently known systems have very short periods
$\simeq0.1-0.3\,{\rm d}$. Here we report the discovery of a low mass companion
($M_{\rm 2}>0.14\,M_{\rm \odot}$) orbiting the sdB star and central star of a
planetary nebula EGB\,5 with an orbital period of $16.5\,{\rm d}$ at a minimum
separation of $23\,R_{\rm \odot}$. Its long period is only just consistent with
the energy balance prescription of the common envelope. The marked difference
between the short and long period systems will provide strong constraints on
the common envelope phase, in particular if the masses of the sdB stars can be
measured accurately. Due to selection effects, the fraction of sdBs with low
mass companions and similar or longer periods may be quite high. Low mass
stellar and substellar companions may therefore play a significant role for the
still unclear formation of hot subdwarf stars. Furthermore, the nebula around
EGB\,5 may be the remnant of the ejected common envelope making this binary a
unique system to study this short und poorly understood phase of binary
evolution.
