  We present a low-energy effective model for the charge degrees of freedom in
two-leg t-J ladders. Starting from SU(2) mean-field theory, we exclude the spin
degrees of freedom which have an energy gap. At low temperatures, the
mean-field solution is the staggered-flux phase. For gapless charge excitations
the effective theory is the Luther-Emery liquid. Our analysis is applicable at
low doping and in the ``physical'' range of parameters $t/J\sim 3$ where there
is only one massless mode in the charge sector and no massless modes in the
spin sector. Within our model we make predictions about correlation exponents
and the superconductivity order parameter, and discuss the comparison with the
existing numerical results.
