  We consider symmetric two-user Gaussian interference channel with common
messages. We derive an upper bound on the sum capacity, and show that the upper
bound is tight in the low interference regime, where the optimal transmission
scheme is to send no common messages and each receiver treats interference as
noise. Our result shows that although the availability of common messages
provides a cooperation opportunity for transmitters, in the low interference
regime the presence of common messages does not help increase the sum capacity.
