2013A&A...555A.133B

We present 69 new mid-eclipse times of the young post-common envelope binary (PCEB) βobject{NNSer}, which was previously suggested to possess two circumbinary planets. We have interpreted the observed eclipse-time variations in terms of the light-travel time effect caused by two planets, exhaustively covering the multi-dimensional parameter space by fits in the two binary and ten orbital parameters. We supplemented the fits by stability calculations for all models with an acceptable χ². An island of secularly stable 2:1 resonant solutions exists, which coincides with the global χ² minimum. Our best-fit stable solution yields current orbital periods P_o=15.47yr and P_i=7.65yr and eccentricities e_o=0.14 and e_i=0.22 for the outer and inner planets, respectively. The companions qualify as giant planets, with masses of 7.0M_Jup and 1.7M_Jup for the case of orbits coplanar with that of the binary. The two-planet model that starts from the present system parameters has a lifetime greater than 10⁸yr, which significantly exceeds the age of NN Ser of 10⁶yr as a PCEB. The resonance is characterized by libration of the resonant variable Θ₁ and circulation of ω_i-ω_o, the difference between the arguments of periapse of the two planets. No stable nonresonant solutions were found, and the possibility of a 5:2 resonance suggested previously by us is now excluded at the 99.3% confidence level.