Mischa Barton -- Chronic Smoker

5/22/2007 3:08 PM PDT BY TMZ STAFF
How does Mischa Barton celebrate making it to Cannes Film Festival?

Well, we're not sure what Mischa's lighting up, but she's no doubt enjoying her time off the French coast ... on designer Roberto Cavalli's yacht. She sure does love those homemade ciggies ...

Perhaps this is what inspired her recent clothing challenge?