If NPVb is the corporate investment criterion, then the corporation will never invest in riskless taxable securities, which pay the riskless before-tax interest rate (and for which X ¼ C0ð1 þ rÞ before tax and C0ð1 þ rð1 2 tcÞÞ after tax), unless there are other cash flow savings involved, e.g. avoiding future fund-raising costs.