  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 by two additional bodies superposed on the linear
ephemeris of the binary. Our multi-parameter fits accompanied by N-body
simulations yield a best fit for the objects NN Ser (ab)c and d locked in a 2:1
mean motion resonance, with orbital periods P_c=15.5 yrs and P_d=7.7 yrs,
masses M_c sin i_c = 6.9 M_Jup and M_d sin i_d = 2.2 M_Jup, and eccentricities
e_c=0 and e_d=0.20. A secondary chi**2 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 M_Sun 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**6 yrs.
