* Follows reports of buyout speculation (Adds details of report, updates shares): FRANKFURT (Reuters) - Private equity firms Bain Capital and Carlyle Group are weighing a joint bid for Germany’s Osram Licht, Bloomberg reported on Wednesday. Bloomberg, which cited unnamed people familiar with the matter, said the two firms were conducting due diligence on Osram and could bid by the end of March but might also decide against a formal offer, Bloomberg reported. Osram, with a market value of around 3.4 billion euros ($3.85 billion), has been the subject of speculation that it could be a buyout target.