article published yes Matthew J Green author Thomas R Marsh author Joannes C van Roestel author 4d122fc8-6083-11f0-87a5-97d68b860333 Tin Long Sunny Wong author Diogo Belloni author Mukremin Kilic author Elmé Breedt author Alex Brown author Chris M Copperwheat author Anurak Chakpor author V S Dhillon author Noel Castro Segura author Martin J Dyer author James Garbutt author Dan Jarvis author Vasu Kengkriangkrai author Mark R Kennedy author Paul Kerry author Thomas Kupfer author S P Littlefair author James McCormac author James Munday author Steven G Parsons author Eleanor Pike author Ingrid Pelisoli author Pablo Rodríguez-Gil author David I Sahman author Amalie Yates author IlCa department Ultracompact binary systems, consisting of two compact objects in an orbit $\lesssim 0.5 {\rm R}_\odot$, should exhibit measurable rates of orbital period change ($\dot{P} \ne 0$) due to the emission of gravitational waves (GWs). Measurements of $\dot{P}$ have so far been limited to the shortest-period ultracompact binaries ($\lesssim 20$  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 $\gtrsim 30$ 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 $\dot{P}$ consistent with zero within $2\sigma$. Their $3\sigma$ upper limits are $1.1 \times 10^{-13}\, {\rm s \, s}^{-1}$ and $9.7 \times 10^{-14}\, {\rm s \, s}^{-1}$, 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 $\dot{P}$ whose time-scales span decades but whose amplitude is $\lesssim 10^{-13}\, {\rm s \, s}^{-1}$. Our non-detections of $\dot{P}$ represent a limit on the strength of any enhanced angular momentum loss beyond pure GW emission. https://research-explorer.ista.ac.at/download/21897/21903/2026_MNRAS_Green.pdf application/pdfno Oxford University Press2026 eng binaries: close – starsdwarf novae – novaecataclysmic variables – white dwarfs 0035-8711 1365-2966 2604.0646010.1093/mnras/stag673 5483 Green, Matthew J, Thomas R Marsh, Joannes C van Roestel, Tin Long Sunny Wong, Diogo Belloni, Mukremin Kilic, Elmé Breedt, et al. “No Period Change in Two Long-Period AM CVn Binaries.” <i>Monthly Notices of the Royal Astronomical Society</i>. Oxford University Press, 2026. <a href="https://doi.org/10.1093/mnras/stag673">https://doi.org/10.1093/mnras/stag673</a>. M. J. Green <i>et al.</i>, “No period change in two long-period AM CVn binaries,” <i>Monthly Notices of the Royal Astronomical Society</i>, vol. 548, no. 3. Oxford University Press, 2026. Green MJ, Marsh TR, van Roestel JC, Wong TLS, Belloni D, Kilic M, Breedt E, Brown A, Copperwheat CM, Chakpor A, Dhillon VS, Segura NC, Dyer MJ, Garbutt J, Jarvis D, Kengkriangkrai V, Kennedy MR, Kerry P, Kupfer T, Littlefair SP, McCormac J, Munday J, Parsons SG, Pike E, Pelisoli I, Rodríguez-Gil P, Sahman DI, Yates A. 2026. No period change in two long-period AM CVn binaries. Monthly Notices of the Royal Astronomical Society. 548(3), stag673. Green, Matthew J., et al. “No Period Change in Two Long-Period AM CVn Binaries.” <i>Monthly Notices of the Royal Astronomical Society</i>, vol. 548, no. 3, stag673, Oxford University Press, 2026, doi:<a href="https://doi.org/10.1093/mnras/stag673">10.1093/mnras/stag673</a>. Green MJ, Marsh TR, van Roestel JC, et al. No period change in two long-period AM CVn binaries. <i>Monthly Notices of the Royal Astronomical Society</i>. 2026;548(3). doi:<a href="https://doi.org/10.1093/mnras/stag673">10.1093/mnras/stag673</a> M.J. Green, T.R. Marsh, J.C. van Roestel, T.L.S. Wong, D. Belloni, M. Kilic, E. Breedt, A. Brown, C.M. Copperwheat, A. Chakpor, V.S. Dhillon, N.C. Segura, M.J. Dyer, J. Garbutt, D. Jarvis, V. Kengkriangkrai, M.R. Kennedy, P. Kerry, T. Kupfer, S.P. Littlefair, J. McCormac, J. Munday, S.G. Parsons, E. Pike, I. Pelisoli, P. Rodríguez-Gil, D.I. Sahman, A. Yates, Monthly Notices of the Royal Astronomical Society 548 (2026). Green, M. J., Marsh, T. R., van Roestel, J. C., Wong, T. L. S., Belloni, D., Kilic, M., … Yates, A. (2026). No period change in two long-period AM CVn binaries. <i>Monthly Notices of the Royal Astronomical Society</i>. Oxford University Press. <a href="https://doi.org/10.1093/mnras/stag673">https://doi.org/10.1093/mnras/stag673</a> 218972026-05-20T14:34:03Z2026-05-21T06:41:41Z