We discuss the impact of the soft degrees of freedom inside the B_c meson
on its rate in the semi-leptonic decay B_c → X l nu_l where X denotes
light hadrons below the D^0 threshold. In particular we identify
contributions involving soft hadrons which are non-vanishing in the limit
of massless leptons. These contributions become relevant for a measurement
of the purely leptonic B_c decay rate, which due to helicity suppression
involves a factor m_l^2 and thus is much smaller than the contributions
involving soft hadrons.