Kay Jewelers Corp., the financially troubled Alexandria jewelry retailer that recently agreed to be taken over by a British firm, said yesterday its loss widened in the second quarter.

The company said it lost $4.4 million in the quarter, compared with a $975,000 loss in 1989's second quarter. Kay typically reports a loss in the first three quarters of the year, but the company said reduced sales and higher interest costs in this year's second quarter helped increase the loss.

Revenue in the period fell 5 percent, to $86.7 million from $91.2 million.

In the first six months of the year, Kay lost $9.98 million, more than double the $4.8 million it lost a year ago. Revenue in the six months slipped 4 percent, to $163.2 million from $170.6 million.

Kay, which operates nearly 500 jewelry stores in the United States under the names Kay Jewelers, J.B. Robinson, Marcus & Co. and Black Starr & Frost, agreed earlier this month to be taken over by Ratners Group PLC of Britain for $350 million. Kay shareholders have yet to approve the transaction, which would create the second-largest jewelry chain in the United States.

Systems Center Inc., a Reston-based company that develops software for IBM mainframe computers, said the costs of its acquisition of an Australian software company led to losses in the second quarter and first half, despite rising revenue.

The company reported a first-half loss of $3.6 million, compared with a profit of $4.6 million (46 cents) in the 1989 first half. Revenue rose 17 percent, to $40 million from $34.1 million.

In the second-quarter, Systems Center lost $4.8 million, compared with a $2.6 million (26 cents) profit in the 1989 second quarter. Revenue increased 25 percent, to $22.9 million from $18.3 million.