NEW YORK • Success is never left alone so the pool of potential Game Of Thrones prequels has expanded.

As HBO's hit show has entered its home stretch, the network has begun developing prequels set in other parts of the vast universe of George R.R. Martin's Song Of Ice And Fire novels, which are the basis for Thrones.

In May, HBO announced that writers were working on four pilot scripts.

This week, details emerged about a fifth concept being developed by Martin and Mr Bryan Cogman, a co-executive producer for Thrones, Martin confirmed on his blog on Wednesday.

"Truth be told, we've had five scripts in various stages of development for months," he wrote.

"But now at last all the deals are signed."

Mr Cogman has been with Thrones since its first season. He joined the production as an assistant to show creators David Benioff and Dan Weiss, before becoming one of its most prolific writers.

He has written more episodes than anyone aside from Mr Benioff and Mr Weiss.

No details have been released about the content of the new Thrones shows, which would be set before the current series and thus include none of the current cast.

Mr Benioff and Mr Weiss have said they will not be involved in any of the new series though they, like Martin, will be executive producers.

HBO has not committed to airing any Thrones successor but it is hard to believe that it would not extend such a lucrative global phenomenon, especially now that the prequel pool includes a Thrones veteran like Mr Cogman and Martin.

The final season of Thrones will probably air in 2019 and any future series would arrive at least a year after that, Mr Casey Bloys, HBO's programming president, has said.

On Thursday, the network confirmed Mr Cogman's project but offered no further information about the prequels, other than to reiterate that any series would not be produced until after Thrones is over.

"You should not expect to see all five shows, though, at least not immediately," Martin wrote on his blog.

"But we could possibly see two or even three make it to the pilot stage, with one series emerging on air in 2019 or 2020... and the others maybe later, if they come out as well as we all hope."

The other writers who have been tapped to develop concepts have never written for Thrones.

In addition to working on three of the five prequel concepts, Martin is also helping to develop Nnedi Okorafor's Afrofuturist novel Who Fears Death into a series for HBO.

In other words, what is good news for Martin's TV fans might be less so for his readers, who have been waiting for the next Ice And Fire novel since 2011.

In July, he wrote that "I am still months away" from completing that book The Winds Of Winter.

"How many? Good question," he said. "I still have good days and bad days, and that's all I care to say."

NYTIMES