a question about the Parisien public transport system. I sometimes seem to buy the wrong ticket i.e. at Gare du Nord I use the machine and select "La Defense" as my destination, but when I get to the other end, the ticket won't let me through the barriers. I want to avoid this now I'm going to be there more often. I know there's no equivalent to the Oyster Card that we have in London, but what would be the best option for someone in my position i.e. will be in Paris every two-three weeks, and need to travel mostly just between Gare du Nord and La Defense, but occasionally into central Paris (AXA Group headquarters is on the Rue Matignon). Is there a book of tickets or something I can buy, and if so, what one do I need (and even more hopefully, how much does it cost?) ta.