Cinematographer-filmmaker Santhosh Sivan will join hands with veteran filmmaker Mani Ratnam for the sixth time as they reunite for a yet-untitled Tamil project, a source said."Yes, Santhosh Sivan is on board Mani Ratnam sir's next film. It's going to be a bilingual but he is yet to finalise his lead actors. The project will roll from September," a source close to Ratnam told IANS.Sivan and Ratnam have previously worked together in films such as "Thalapathi", "Dil Se", "Iruvar", "Roja" and "Raavanan".To be bankrolled by Ratnam's Madras Talkies, the film will have music by A.R. Rahman.The source also clarified that actor Ram Charan is not part of the project."Although Ram Charan and Ratnam had met and discussed the idea of collaborating, the plan didn't materialise. He is not part of this project," the source added.