Detection of a period decrease in NN Ser with ULTRACAM: evidence for strong magnetic braking or an unseen companion

We present results of high time resolution photometry of the eclipsing pre-cataclysmic variable NN Ser. NN Ser is a white dwarf/M dwarf binary with a very low-mass secondary star (∼0.2 M ○. ). We observed 13 primary eclipses of NN Ser using the high-speed CCD camera ULTRACAM and derived times of mid-eclipse, from fitting of light-curve models, with uncertainties as low as 0.06 s. The data show that the period of the binary is decreasing, with an average rate of P = (9.06 ± 0.06) x 10 -12 s s -1 , which has increased to a rate of P = (2.85 ± 0.15) x 10 -11 s s -1 over the last 2 yr. These rates of period change appear difficult to reconcile with any models of orbital period change. If the observed period change reflects an angular momentum loss, the average loss rate (J = 1.4 ± 0.6 x 10 35 erg) is consistent with the loss rates (via magnetic stellar wind braking) used in standard models of close binary evolution, which were derived from observations of much more massive cool stars. Observations of low-mass stars such as NN Ser's secondary predict rates of ∼ 100 times lower than we observe. The alternatives are either magnetic activity-driven changes in the quadrupole moment of the secondary star (Applegate's mechanism) or a light traveltime effect caused by the presence of a third body in a long (∼ decades) orbit around the binary. We show that Applegate's mechanism fails by an order of magnitude on energetic grounds, but that the presence of a third body with mass 0.0043 < M 3 < 0,18 M ○. and orbital period 30 < P 3 < 285 yr could account for the observed changes in the timings of NN Ser's mid-eclipses. We conclude that we have either observed a genuine angular momentum loss for NN Ser, in which case our observations pose serious difficulties for the theory of close binary evolution, or we have detected a previously unseen low-mass companion to the binary.