Stock index futures slipped on Monday after Standard & Poor's placed its long-term sovereign ratings on 15 euro zone countries on CreditWatch with negative implications.

S&P 500 futures fell 0.5 point and were in line with fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 6 points and Nasdaq 100 futures lost 1 point.

(Reporting by Angela Moon, Editing by Kenneth Barry)