We observe near band edge photoluminescence (PL) from pseudomorphic Si/Si1-yCy multiple quantum wells and thick Si1-yCy epilayers, which were grown by molecular beam epitaxy on Si substrates. Pieces of these structures were annealed at temperatures between 500 and 1000 degrees C to investigate their thermal stability. While annealing at 900 degrees C or more leads to a reduction of the amount of substitutionally incorporated carbon and to a quenching of the PL signal, annealing at temperatures between 500 and 700 degrees C results in increased PL intensities and reduced Linewidths. Although no decrease in the carbon content is observed under these conditions, we also observe a blue shift of the PL lines after annealing. In addition, increasing the temperature, at which the PL is recorded, leads to a red shift of the lines. We show that these findings are consistent with a recombination model of excitons bound to alloy fluctuations and a homogenization of not statistically distributed carbon atoms upon annealing.