  Transition towards spatio-temporal chaos in one-dimensional interfacial
patterns often involves two degrees of freedom: drift and out-of-phase
oscillations of cells, respectively associated to parity breaking and
vacillating-breathing secondary bifurcations. In this paper, the interaction
between these two modes is investigated in the case of a single domain
propagating along a circular array of liquid jets. As observed by Michalland
and Rabaud for the printer's instability \cite{Rabaud92}, the velocity $V_g$ of
a constant width domain is linked to the angular frequency $\omega$ of
oscillations and to the spacing between columns $\lambda_0$ by the relationship
$ V_g = \alpha \lambda_0 \omega$. We show by a simple geometrical argument that
$\alpha$ should be close to $1/ \pi$ instead of the initial value $\alpha =
1/2$ deduced from their analogy with phonons. This fact is in quantitative
agreement with our data, with a slight deviation increasing with flow rate.
