ya debenture issue involves cost. I think all these bank has issue debenture at 8%. So this interest amount is deductible from its revenue (treated as interest expenses heading of Profit and Loss account).
Definitely it involves some cost and effect profit negatively. But we need to see the cost and benefit of this debenture issued amount. If this issue cost 8% and if this amount is yielding more than 8% then this will effect profit positively which means increased ROE.