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

摘要

Planets orbiting post-common envelope binaries provide fundamentalinformation on planet formation and evolution. We searched for suchplanets in NNSerab, an eclipsing short-period binary thatshows long-term eclipse time variations. Using published, reanalysed,and new mid-eclipse times of NNSerab obtained between 1988and 2010, we find excellent agreement with the light-travel-time effectproduced by two additional bodies superposed on the linear ephemeris ofthe binary. Our multi-parameter fits accompanied by N-bodysimulations yield a best fit for the objects NNSer(ab)c andd locked in the 2:1 mean motion resonance, with orbital periods yrs and yrs, masses and ,and eccentricities and .A secondary minimum corresponds to an alternative solution with a period ratio of5:2. We estimate that the progenitor binary consisted of an A star withand the present Mdwarf secondary at an orbital separation of 1.5AU.The survival of two planets through the common-envelope phase thatcreated the present white dwarf requires fine tuning between thegravitational force and the drag force experienced by them in theexpanding envelope. The alternative is a second-generation origin in acircumbinary disk created at the end of this phase. In that case, theplanets would be extremely young with ages not exceeding the coolingage of the white dwarf of 106yrs. Key words: binaries: eclipsing - planetary systems - stars: individual: NN Ser - novae - cataclysmic variables -planets and satellites: formation - stars: evolution