Banca Monte dei Paschi di Siena, the oldest surviving bank in the world, is working on a rights issue to fill at least half of a 2.1 billion euro ($2.6 billion) capital gap uncovered by a European review of banks, a source with direct knowledge of the matter said. Italy’s third-largest bank has hired UBS and Citigroup to assess strategic options after it failed European Central Bank (ECB) check tests designed to test the solidity of the financial system.