6H- and 15R-SiC crystals were enriched by (13)C during the sublimation growth in a modified Lely furnace. Furthermore, a (13)C enriched 6H-SiC sample was implanted with helium to generate the D(1)-defect. The samples have been investigated by low temperature photoluminescence (LTPL). The helium implanted samples were additionally studied by photoluminescence (PL) at temperatures up to 59 R. We report on an isotopic effect in 6H- and 15R-SiC on the momentum-conserving optical phonon lines of the exciton recombination as well as on the zero phonon exciton recombination lines. We estimate a shift to higher energy of the zero-phonon bound exciton lines of a 6H-SiC crystal with pure (13)C by 2.19 meV for nitrogen and 0.85 meV for titanium relative to a sample with natural isotopic composition.