A once-in-a-lifetime opportunity for euro zone governments to borrow for half a century or more at rock-bottom interest rates might be coming to an end.
A chorus of state debt office chiefs, bank dealers and investors are warning that sharp rises in long-term bond yields in recent months could make it hard for countries to repeat the unprecedented volume of 50-, 70- and 100-year debt sold in 2016.
The era of ultra-low rates has been a boon for euro zone governments, allowing them to fund their budget deficits remarkably cheaply since the global financial crisis; a number have now locked in funds for very long periods, on top of the more traditional 10- to 30-year maturities.
But this "golden moment" for governments hasn't necessarily extended to investors, especially since Donald Trump's U.S. election victory prompted a bond market sell-off.
Not only are some funds still nursing around 10 percent losses on some of their long-bond investments, but the demand and the financial logic behind these sales may be waning.
Even Belgium, which said in its 2017 funding plan that it was exploring the possibility of launching a "new, very long-term benchmark", seems unsure.
"Next year it might be more difficult to do something in the longer end, given the possible immediate gain that an investor can make is not foreseeable," said Anne Leclercq, head of the Belgian debt office which manages the country's bond issuance.
Belgium launched a multi-billion euro 50-year bond in 2016, as did Spain, Italy and France. Austria issued a 70-year bond in October and has started the legislative process to permit issuance of up to 100 years to match smaller private placements sold by Ireland and Belgium in 2016.
But for the investors who bought those bonds, from pension schemes to hedge funds, the results have been mixed.
Current trading levels on the Austrian and Italian debt suggest around a 10 percent loss, while France's bond has returned around 1 percent, and Belgium and Spain's offer an 8 and 10 percent profit, respectively, according to Reuters data.
All have shed value since Trump's presidential victory in November, which set off the drop in bond prices on expectations of higher growth and inflation in the world's largest economy. Those losses deepened after the European Central Bank this month scaled back the pace of its planned bond purchases under its program of monetary stimulus.
The yield on Europe's benchmark German 30-year bond hit an 11-month high after the ECB meeting, up nearly half a percentage point from before the U.S. election.
Some analysts say the fact that some of these bonds are still trading at a profit means more deals could come early next year - at least until the focus shifts to the possibility of further reductions in the ECB's bond-buying program due to expire next December.
"If you want to do it, do it in the first half of next year," ING strategist Martin van Vliet, said. "In the second half, the tapering talk will revive and the door will be nearly fully closed by then."
For borrowers, the rationale for ultra-long issuance could fade quickly as rates rise. Already some analysts have questioned the cost of raising this debt, saying it would be cheaper for a government to issue, for example, 30-year bonds and then return to the market when they mature.
A broad range of investors have poured money into ultra-long bonds in the past year, hoping they would offer better returns than the record low rates on more conventional maturities. Asset managers joined the pension and insurance funds which traditionally buy long-dated bonds to match their liabilities.
Then there were the hedge funds looking for a quick buck on these bonds that are vulnerable - especially at very low yields - to sharp price swings, and investors who bought them to hedge against the risk of a Japan-style decade of deflation.
But bankers who sold these landmark deals told Reuters, on condition of anonymity, that demand may be drying up as interest rates generally start to rise.
"The fact that rates are higher reduces the needs for some real money investors for these products. They were forced to go longer to get yield," one banker said. "Conditions should be less conducive for long-dated issuance now."
JPMorgan Asset Management has already cut its exposure to ultra-long debt and, for the first time in two years, has a bias towards shorter-dated bonds.
"We would be cautious about those bonds - if we are in an environment where yields are going higher, those are the types of bonds that are particularly sensitive," its chief investment officer for fixed income, Nick Gartside, said.
The world's biggest asset manager BlackRock, while saying it would not rule out buying ultra-long debt, is also cautious about what its head of global bonds Scott Thiel called the "double whammy" of very low yields and longer duration.
"Countries are doing the right thing by issuing long-dated bonds when there's a lower level of interest rates," said Thiel. "But when you get to a very long-dated bond, the return is currently so low you have to be careful."