In which TV series is Penny a main character?

Who played Joan Holloway in 'Mad Men'? In which TV series is Joan Holloway a main character? Who played Leonard Hofstadter in 'The Big Bang Theory'? Who played Penny in 'The Big Bang Theory'? Who played Howard Wolowitz in 'The Big Bang Theory'? Who played Raj Koothrappali in 'The Big Bang Theory'? Who played Blair Waldorf in 'Gossip Girl'? In which TV series is Blair Waldorf a main character?