  We formulate charge transfer model for a single doped spin ladder, relevant
for the intrinsically doped plane of coupled spin ladders in
Sr$_{3}$Ca$_{11}$Cu$_{24}$O$_{41}$. Assuming the presence of the experimentally
observed charge order in the system we solve the model using Hartree-Fock
approximation. Our results show the profound stability of the Zhang-Rice
singlets with respect to other configurations of doped holes, in agreement with
recent x-ray absorption measurements.
