  We consider the effect of broadband decoherence on the performance of
refocusing sequences, having in mind applications of dynamical decoupling in
concatenation with quantum error correcting codes as the first stage of
coherence protection. Specifically, we construct cumulant expansions of
effective decoherence operators for a qubit driven by a pulse of a generic
symmetric shape, and for several sequences of $\pi$- and $\pi/2$-pulses. While,
in general, the performance of soft pulses in decoupling sequences in the
presence of Markovian decoherence is worse than that of the ideal
$\delta$-pulses, it can be substantially improved by shaping.
