No Period Change in Two Long-Period AM CVn Binaries

Ultracompact binary systems, consisting of two compact objects in an orbit ≲ 0.5R⊙, should exhibit measurable rates of orbital period change (Ṗ ≠ 0) due to the emission of gravitational waves (GWs). Measurements of Ṗ have so far been limited to the shortest-period ultracompact binaries (≲ 20 min). Among the AM CVn-type subclass, several works have proposed the presence of extra angular momentum loss beyond GW emission, with magnetic braking being a widely discussed mechanism. If present, this magnetic braking would dominate the angular momentum loss of AM CVn-type binaries with orbital periods ≳ 30 min. In this work, we present a long-term eclipse timing study of two AM CVn-type binaries, YZ LMi and Gaia14aae, with respective orbital periods of 28.3 min and 49.7 min and continuous observations since 2006 and 2015. Both systems show Ṗ consistent with zero within 2σ. Their 3σ upper limits are 1.1 × 10−13 s s−1 and 9.7 × 10−14 s s−1 respectively. These non-detections are most simply explained by a scenario in which secular angular momentum loss is not substantially stronger than GW emission at all orbital periods, but is combined with deviations from the secular Ṗ whose timescales span decades but whose amplitude is ≲ 10−13 s s−1. Our non-detections of Ṗ represent a limit on the strength of any enhanced angular momentum loss beyond pure GW emission.