  We combine measurements of the top quark pair production cross section in
ppbar collisions in the l+jets, ll and tau-l final states (where l is an
electron or muon) at a center of mass energy of sqrt(s)=1.96 TeV in 1fb^{-1} of
data collected with the D0 detector. For a top quark mass of 170 GeV, we obtain
sigma_ttbar=8.18^{+0.98}_{-0.87} pb. In addition, the ratios of ttbar cross
sections in different final states are used to set upper limits on the
branching fractions B(t -> H^{+}b -> tau nu b) and B(t -> H^{+}b -> c sbar b)
as a function of charged Higgs boson mass. Based on predictions from higher
order quantum chromodynamics, we extract a mass for the top quark from the
combined ttbar cross section.
