I also think that you should try to find out if she is lost. If not then I'd keep her as long as you can handle another cat. I'm at my limit with 3 but if I found a needy stray cat I'd keep it at least until I was able to find a loving home for it. I think that maybe someone knew that you had a lot of black& white cats so they dropped it off at your house and hoped that you would think it was one of yours or that you would adopt it. Good luck. Please keep us posted.