We present 69 new mid-eclipse times of the young post-common envelope binary ( PCEB ) NN Ser , 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 \chi ^ { 2 } .
An island of secularly stable 2 : 1 resonant solutions exists , which coincides with the global \chi ^ { 2 } minimum .
Our best-fit stable solution yields current orbital periods P _ { \mathrm { o } } = 15.47 yr and P _ { \mathrm { i } } = 7.65 yr and eccentricities e _ { \mathrm { o } } = 0.14 and e _ { \mathrm { i } } = 0.22 for the outer and inner planets , respectively .
The companions qualify as giant planets , with masses of 7.0 M _ { \mathrm { Jup } } and 1.7 M _ { \mathrm { 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 10 ^ { 8 } yr , which significantly exceeds the age of NN Ser of 10 ^ { 6 } yr as a PCEB .
The resonance is characterized by libration of the resonant variable \Theta _ { 1 } and circulation of \omega _ { \mathrm { i } } - \omega _ { \mathrm { 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 .