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

NN Ser has long been known as a common-envelope eclipsing binary consisting of a hydrogen-rich white dwarf, NN Ser a, and an M4 red dwarf, NN Ser b. By monitoring the eclipses observed since 1988, Bauerman et al. find that the system is more complex. Deviations from the predicted eclipse timings imply that it should contain at least two more circumbinary planets: NN Ser (ab)c with mass Msini~6.9 Mjup and a 15.5yr orbital period, and NN Ser (ab)d with mass Msini~2.2 Mjup and a 7.7 yr orbital period (*). This is the second such system discovered after HW Vir, another very short eclipsing binary found to be orbited by two massive giant planets or brown dwarfs. It raises the problem of either the survival or formation of these objects. (*) In the absence of an adopted naming convention for circumbinary planets, temporary names were derived with the following reasoning: the two stars form a spectroscopic binary, usually named with lower-case letters, i.e. NN Ser a and b. Because the two planets are on the same hierarchical level as the stars (they all orbit around the same center of mass), they were also named with lower case letters, c and d. Parentheses were used to stress their particular nature, i.e. that they orbit around the two stars a and b. The two planets were therefore named NN Ser (ab)c and NN Ser (ab)d.