It’s alive, but for how long? That was the prognosis on the box office birth of Mary Shelley’s Frankenstein, directed by and starring Kenneth Branagh. Taking in $11.2 million, it didn’t fly like Bram Stoker’s Dracula did two Novembers ago ($30 million in its first three days) — and vaunted competitor Interview With the Vampire waits in the wings. Meantime, sci-fi sleeper StarGate may make a bigger killing than either creature feature: Its gross dropped only 25 percent from last week’s No. 1 opening. Kevin Costner scored no such victory with The War, which barely beat Pulp Fiction for third place but could outgun his misfires Wyatt Earp and A Perfect World.