FRANKFURT/MADRID (Reuters) - General Electric <GE.N> and German wind turbine maker Senvion <SENG.DE> are preparing bids for French wind power group Adwen, which is jointly owned by Spain's Gamesa <GAM.MC> and France's Areva <AREVA.PA>, people familiar with the matter said.

German industrials group Siemens <SIEGn.DE> is due to take over Gamesa's 50 percent stake in Adwen as part of its 1 billion euro ($1.1 billion) deal to buy a majority stake in Gamesa. It has also made an offer for Areva's stake in Adwen.

However, under the terms of a side-deal agreed in connection with Siemens's buyout of Gamesa, Areva has until mid-September to look for an alternative buyer.

The companies declined to comment.

Siemens Chief Executive Joe Kaeser said last month he believed his company's offer for Adwen was compelling.

"We took a lot of time to discuss the offshore projects in France with customers, where the risks are, where benefits could be, on so-called legacy projects that obviously have their challenges," he said.

"If we get the 50 percent that is fine, and if we don't, if someone puts a better offer there, they maybe deserve it."

Gamesa values its 50 percent equity stake in Adwen at 74 million euros, according to its 2015 annual report.

"Areva has a put option to sell its 50 percent stake to Gamesa for 60 million euros. I would be surprised if the premium offered would be massive," one of the people familiar with the matter said.

Adwen's products include an 8 megawatt offshore wind turbine, a machine with the largest annual energy production in the industry, for which the group has almost 200 orders already, according to the company website.

"But it's a prototype with production starting in two years time, and the orders aren't that fixed," one of the people said.

The merged Siemens-Gamesa wind company will have a market capitalization of around 10 billion euros, according to analysts, and would overtake Denmark's Vestas <VWS.CO> to become the world's biggest builder of wind farms by market share.

Vestas is not interested in buying Adwen, a company spokesman said.

(Reporting by Arno Schuetze, Jose Elías Rodríguez and Andres Gonzalez; Additional reporting by Georgina Prodhan, Teis Jensen and Benjamin Mallet; Editing by Mark Potter)