  We calculate non-perturbatively the coefficient c_sw required for O(a)
improvement of the SU(3) gauge theory with Nf = 2 fermions in the two-index
symmetric (sextet) representation. For the calculations we impose the standard
improvement condition in the Schr\"odinger functional framework, using two
different discretisations for the gauge field i.e. the Wilson plaquette action
and the tree-level improved Symanzik (L\"uscher-Weisz) action.
