  We present 25 new eclipse times of the white dwarf binary NN Ser taken with
the high-speed camera ULTRACAM on the WHT and NTT, the RISE camera on the
Liverpool Telescope, and HAWK-I on the VLT to test the two-planet model
proposed to explain variations in its eclipse times measured over the last 25
years. The planetary model survives the test with flying colours, correctly
predicting a progressive lag in eclipse times of 36 seconds that has set in
since 2010 compared to the previous 8 years of precise times. Allowing both
orbits to be eccentric, we find orbital periods of 7.9 +/- 0.5 yr and 15.3 +/-
0.3 yr, and masses of 2.3 +/- 0.5 Mjup and 7.3 +/- 0.3 Mjup. We also find
dynamically long-lived orbits consistent with the data, associated with 2:1 and
5:2 period ratios. The data scatter by 0.07 seconds relative to the best-fit
model, by some margin the most precise of any of the proposed eclipsing compact
object planet hosts. Despite the high precision, degeneracy in the orbit fits
prevents a significant measurement of a period change of the binary and of
N-body effects. Finally, we point out a major flaw with a previous dynamical
stability analysis of NN Ser, and by extension, with a number of analyses of
similar systems.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics