Map of modified chi-square, , as a function of the two orbital periods of the two-circular-planets model for HD 27894. All models on this map are stable. Contours are combined confidence levels with respect to the best fit, i.e. the 1, 2, 3 and 4σ regimes in which both period values are found together with a probability of 68.27%, 95.45%, 99.73%, and 99.99%, respectively (while the remainder of the orbital parameters is optimized, i.e. treated as uninteresting). The best fit with is denoted by an asterisk. It is within the ± 9% region from the exact 2:1 period ratio. The left and right slanted dashed lines delimit this region whereas the middle one marks the exact ratio. Exact 2:1 solutions are found within the 3σ contour (middle). The 1, 2, 3 and 4σ contours correspond to , and 32.33, respectively.