Two planets orbiting the recently formed post-common envelope binary NN Serpentis

Abstract

Planets orbiting post-common envelope binaries provide fundamental information on planet formation and evolution. We searched for such planets in NN Ser ab, an eclipsing short-period binary that shows long-term eclipse time variations. Using published, reanalysed, and new mid-eclipse times of NN Ser ab obtained between 1988 and 2010, we find excellent agreement with the light-travel-time effect produced by two additional bodies superposed on the linear ephemeris of the binary. Our multi-parameter fits accompanied by <i>N<i/>-body simulations yield a best fit for the objects NN Ser (ab)c and d locked in the 2:1 mean motion resonance, with orbital periods <i>P<i/><sub>c<sub/> 15.5 yrs and <i>P<i/><sub>d<sub/> 7.7 yrs, masses <i>M<i/><sub>c<sub/> sin <i>i<i/><sub>c<sub/> 6.9 <i>M<i/><sub>Jup<sub/> and <i>M<i/><sub>d<sub/> sin <i>i<i/><sub>d<sub/> 2.2 <i>M<i/><sub>Jup<sub/> and eccentricities <i>e<i/><sub>c<sub/> 0 and <i>e<i/><sub>d<sub/> 0.20. A secondary <i>χ<i/><sup>2<sup/> minimum corresponds to an alternative solution with a period ratio of 5:2. We estimate that the progenitor binary consisted of an A star with ~2 and the present M dwarf secondary at an orbital separation of ~1.5 AU. The survival of two planets through the common-envelope phase that created the present white dwarf requires fine tuning between the gravitational force and the drag force experienced by them in the expanding envelope. The alternative is a second-generation origin in a circumbinary disk created at the end of this phase. In that case, the planets would be extremely young with ages not exceeding the cooling age of the white dwarf of 10<sup>6<sup/> yrs.