A complex form of European bank debt has made a comeback in recent weeks, in the latest sign that investors are willing to take on higher amounts of risk in their search for positive returns.

Barclays added to a string of recent sales of so-called contingent convertible-or CoCo-bonds Wednesday. These securities fell sharply out of favour with investors earlier this year, when there were concerns that German lender Deutsche Bank might fail to make interest payments on some of its bonds. It made the payments.