The Big Bang Theory- what is Penny's last name?

I have been looking for an answer online but haven't found the answer. Some people say it could be Johnson, Johnston or Johnstone. Other names I have seen suggested are Barrington (supposedly the creator of the show claims this is her last name), Longbottom, Postelwaite and Newton. I'm sure there are more, but I have only looked at a couple of sites.

Her parents names are Wyatt and Susan. She has a sister but her name is never mentioned, and she has a brother named Randall.

http://bigbangtheory.wikia.com/wiki/Penny_Hofstadter

Well that is a good question and something I have not even thought of before. I guess I just never realized that they had never mentioned her last name on the show.

I think it actually is Barrington.

I thought they intentionally never mention her last name, but always wondered about that.

Penny Barrington? She doesn't look like a Barrington, but then again she doesn't look much like a Penny either.