We explore the complementarity between a central bank and a financial stability Fund in stabilizing sovereign debt markets. The central bank pursuing its mandate can intervene with public sector purchasing programs, buying sovereign debt in the secondary market, provided that the debt is safe. The sovereign sells its debt to private lenders, through market auctions. Furthermore, it has access to a long-term state-contingent contract with a Fund: a country-specific debt-and-insurance contract that accounts for no-default and no-over-lending constraints. The Fund needs to guarantee gross-financial-needs and no-over-lending. We show that these constraints endogenously determine the ‘optimal debt maturity’ structure that minimizes the Required Fund Capacity (RFC) to make all sovereign debt safe. […]