  We theoretically study Coulomb drag between two helical edges with broken
spin-rotational symmetry, such as would occur in two capacitively coupled
quantum spin Hall insulators. For the helical edges, Coulomb drag is
particularly interesting because it specifically probes the inelastic
interactions that break the conductance quantization for a single edge. Using
the kinetic equation formalism, supplemented by bosonization, we find that the
drag resistivity $\rho_D$ exhibits a nonmonotonic dependence on the temperature
$T$. In the limit of low $T$, $\rho_D$ vanishes with decreasing $T$ as a power
law if intraedge interactions are not too strong. This is in stark contrast to
Coulomb drag in conventional quantum wires, where $\rho_D$ diverges at $T\to 0$
irrespective of the strength of repulsive interactions. Another unusual
property of Coulomb drag between the helical edges concerns higher $T$ for
which, unlike in the Luttinger liquid model, drag is mediated by plasmons. The
special type of plasmon-mediated drag can be viewed as a distinguishing feature
of the helical liquid---because it requires peculiar Umklapp scattering only
available in the presence of a Dirac point in the electron spectrum.

Mesoscale and Nanoscale Physics;Strongly Correlated Electrons