Halcyon Investments, a New York-based partnership, said last week it had agreed to sell its 5.2 percent stake in Atlantic Research Corp. to Sequa Corp. for $10.15 million, a deal that will boost Sequa's interest in the company from 12.3 percent to 17.5 percent.

The sale could mean a loss of about $5.3 million to Halcyon, which bought the 483,400-share stake in Atlantic Research for $15.42 million.

According to a filing with the Securities and Exchnge Commission, the sale is subject to federal regulatory approval.

Halcyon decision to unload its stake in Atlantic Research apparently marks the end of earlier plans to acquire the company.

In September, Halcyon said it had met with William Borten, president of Atlantic Research, to discuss its investment and the company's business prospects.

Halcyon also said at the time that it planned to meet with other shareholders of the company, possibly to discuss an acquisition proposal for Atlantic Research, an Alexandria-based maker of rockets and data communication equipment.

New York-based Sequa is a diversified company with interests in graphic equipment, electro-optical systems, metal fabrication, transportation, apparel and petroleum services.

In previous filings with the SEC ,Sequa said it had acquired Atlantic Research stock for investment purposes.

Sequa agreed in September to buy 12.3 percent of Atlantic Research's stock for $34.2 million from Clabir Corp. and Clabir's subsidiary, General Defense Corp.

Earlier in the year Clabir, a Greenwich, Conn.-based holding company, and General Defense, a defense contractor, failed in an attempt to take over Atlantic Research.