Results of fitting the Keplerian two-planet model to the eclipse-time data of NN Ser. Left: χ² distribution in the e_o, e_i plane. The + sign indicates the minimum  χ² and the contour lines refer to the increments Δ   χ² indicated in the legend. Center: maximum lifetime τ of models with χ² at the 99.9% confidence level (Δ   χ² =  + 13.8 for two degrees of freedom). For ease of comparison, the  χ² contours of the left panel are included. Right: same for models with χ² at the 68.3% confidence level (Δ   χ² =  + 2.3).