  We study the efficiency at maximum power, $\eta^*$, of engines performing
finite-time Carnot cycles between a hot and a cold reservoir at temperatures
$T_h$ and $T_c$, respectively. For engines reaching Carnot efficiency
$\eta_C=1-T_c/T_h$ in the reversible limit (long cycle time, zero dissipation),
we find in the limit of low dissipation that $\eta^*$ is bounded from above by
$\eta_C/(2-\eta_C)$ and from below by $\eta_C/2$. These bounds are reached when
the ratio of the dissipation during the cold and hot isothermal phases tend
respectively to zero or infinity. For symmetric dissipation (ratio one) the
Curzon-Ahlborn efficiency $\eta_{CA}=1-\sqrt{T_c/T_h}$ is recovered.
