been 'adopted' by very cute cat - but what to do?

[fullmoonfiend]

We have a lovely 10-yr-old black mog. But The last 2 weeks, a small, black female (I think) keeps turning up on my bed, ds's bed, the wardrobe etc. She's very friendly and appealing and seemingly in good nick so I doubt she's a stray. We can't shut the cat flap (as our mog needs it) and the kids have thwarted all my best efforts to be stern, though I do keep throwing her out. I'm so aware of the fact tht she must belong to someone, but we live in a densely populated area so no idea how to go about tracing her real oweners.
My cat doesn't seem very threatend - he growls a bit but I've seen him really go for other cats who've wandered onto his patch. >
If not any bright ideas how to find out where she comes from? The only thing I can think of is to buy her a collar and attatch a litle baggy with a note to her owners, explaining the situation.
She's so cute...not sure I can resist much longer without feeding her

It sounds as if your Mog has accepted the new one! If mog wasn't happy then he'd have seen the new one off by now. I'd go with the collar idea and see what happens. Sounds like you've been adopted

can you put up a poster in the local area? also, have a look at your local shops to see if someone has lost a cat? One other thing, if you're planning to keep her, then you should take her to the vet and check her boosters are up to date, and that she's been spaid, you don't want kittens.