Ad giant Publicis to buy Digitas to expand online

Publicis Groupe, the world’s fourth-biggest advertising firm, agreed to buy Boston-based Digitas Inc. for $1.3 billion in cash to expand in online marketing.

Paris-based Publicis will offer $13.50 for every Digitas share, a 27% premium to Wednesday’s share price, the company said in an e-mailed statement. Publicis shares rose 87 cents to $40.51.