Crestar Financial Corp., citing weakening real estate markets and the need to strengthen its reserve against possible loan losses, yesterday reported a 21 percent decline in profit for the first nine months of the year.

The Richmond-based bank company, parent of the area's seventh-largest financial institution, said it earned $60.2 million, or $1.87 a share, compared with a profit of $76.4 million ($2.42 a share) in the same nine months a year earlier.

For the third quarter, Crestar's profit fell 9 percent, to $24.2 million (76 cents) from $26.6 million (83 cents) in the 1989 third quarter. Its total assets stood at $12.7 billion at the end of the quarter.

Troubled real estate loans are increasing for Crestar; the bank company said it added $19.7 million to its loan-loss reserve in the third quarter after adding $25 million to the fund in the second period.

Its nonperforming assets -- mostly loans that are past due or are in default, and real estate Crestar now owns -- rose to $133.4 million as of Sept. 30, from $119.1 million three months earlier.

Nonperforming assets totaled 1.68 percent of Crestar's total loans as of Sept. 30, compared with 1.52 percent three months earlier.

Its write-offs on bad debts were up to $15.3 million, or 0.79 percent of average loans, at the end of the quarter, from $10.1 million (0.53 percent) at the end of the same period last year.

Offsetting the additions to loan-loss reserves, Crestar's net interest income rose 12 percent in the third quarter, primarily due to boosts in investment securities and money market investments, it said.

Non-interest income also rose 5 percent, and securities gains were $3.4 million higher than in the year-ago third quarter.

Orbital Sciences Corp., the Fairfax-based space technology company, reported a third-quarter profit and narrowed a loss in the first nine months as revenue continued to surge on sales of its Pegasus rocket.

In the first nine months, Orbital lost $535,000, compared with a $5.4 million loss in the first nine months of 1989. Revenue rose 33 percent in the first nine months, to $75.9 million from $56.8 million in the same period a year earlier.

The nine-month loss reflected the remaining development costs that Orbital is paying on the Pegasus rocket, which is carried aloft by aircraft and then launched into space with satellite payloads. But the Pegasus program became profitable in the third quarter and accounted for much of Orbital's gains, Orbital said. It also cited work on the Minuteman missile program.

The company said it won several new orders for products and services during the third quarter, including being chosen for talks with the Strategic Defense Initiative Organization on a contract to develop a suborbital booster system.

O'Sullivan Corp., a Winchester, Va.-based plastics manufacturer that supplies automakers, said its profit rose almost 36 percent in the third quarter but fell nearly 8 percent in the first nine months of the year.

The company reported a third-quarter profit of $3.26 million (20 cents), compared with earnings of $2.4 million (15 cents) in the 1989 third quarter. Revenue increased 2 percent in the quarter, to $49 million from $48 million.

O'Sullivan earned $11.2 million (68 cents) in the first nine months, compared with a profit of $12.1 million (74 cents) in the same period a year earlier. Revenue fell 8 percent in the first nine months, to $154.6 million from $168.4 million in the same period a year ago.