FRANKFURT, Dec 16 (Reuters) - German securities watchdog BaFin opened an official investigation into trading of Porsche shares prior to the sports car maker's investment in larger peer Volkswagen , the regulator said on Friday. "We're examining conspicuous movements in the share price of Porsche before the company stated it had bought a stake in Volkswagen," a spokeswoman for BaFin said, adding: "It has to do with transactions before the drop in the (Porsche) share price."