  We investigate stau-antistau annihilation into heavy quarks in the
phenomenological Minimal Supersymmetric Standard Model within the DM@NLO
project. We present the calculation of the corresponding cross section
including corrections up to $\mathcal{O}(\alpha_s)$ and QED Sommerfeld
enhancement. The numerical impact of these corrections is discussed for the
cross section and the dark matter relic density, where we focus on top-quark
final states and consider either neutralino or gravitino dark matter. Similarly
to previous work, we find that the presented corrections should be included
when calculating the relic density or extracting parameters from cosmological
observations. Considering scheme and scale variations, we estimate the
theoretical uncertainty that affects the prediction of the annihilation cross
section and thus the prediction of the relic density.
