A buyout firm with a record of taking troubled retailers private disclosed on Tuesday that an affiliate had taken an 8 percent stake in Aeropostale, sending the struggling teen apparel retailer's shares up as much 19.5 percent.

Between August 23 and September 11, Sycamore affiliate Hummingbird LLC bought 6.25 million Aeropostale shares at prices ranging from $8.2723 to $8.7564 each, according to a filing with the U.S. Securities and Exchange Commission.

"We believe Hummingbird will attempt to take the company private," Stifel Nicolaus analyst Richard Jaffe said in a research note.

A spokesman for Sycamore declined to comment on the firm's plans for Aeropostale.

The retailer's shares were trading at $10.06 at mid-morning, giving the company a market value of $789.6 million. Earlier in the session they reached as high as $10.29.

Sycamore, which has $1 billion under management, earlier this year took teen retailer Hot Topic Inc private for $600 million, and last year bought women's apparel chain Talbots Inc for $391 million, including debt.

Aeropostale last month reported a decline in second-quarter sales and forecast a deep third-quarter loss, pointing to a highly promotional environment that has forced it to cut prices. Its efforts to offer more fashionable merchandise beyond its hoodies, jeans and t-shirts have fallen flat.