article published yes James Munday author Antonio C. Rodriguez author Nicole Reindl author Nina Mackensen author Tin Long Sunny Wong author S. P. Littlefair author Ingrid Pelisoli author Alex Brown author N. Castro Segura author Joheen Chakraborty author Harry Dawson author V. S. Dhillon author Matti Dorsch author Martin Dyer author James A. Garbutt author Stephan Geier author Matthew Green author Dan Jarvis author Mark R. Kennedy author Paul Kerry author James Mccormac author Steven G. Parsons author Joannes C van Roestel author 4d122fc8-6083-11f0-87a5-97d68b860333 Dave Sahman author P. E. Tremblay author Amalie Yates author IlCa department The AM CVn class includes mass transferring, ultra-compact double white dwarf binaries with orbital periods on the timescale of minutes. A long-standing puzzle is that none of the roughly fifty ultra-compact, ‘verification binaries’ that are easily detectable in the millihertz gravitational wave regime reside in a triple star configuration. Much evidence has hinted at V407 Vul being an inspiraling, double white dwarf AM CVn with an orbital period of 569 s. Yet, a decisive confirmation has proved challenging since a main sequence star dominates its visible spectrum. We present a clear confirmation of the triple star nature of the source by detecting a significant astrometric wobble of the photocentre on the 569 s orbital period of the binary. The AM CVn and the main sequence components are gravitationally bound with a spatial separation of roughly 0.03–0.04″, equating to an orbital separation of approximately 120 AU. A total of 23 years of orbital timing constrained the orbital decay of the AM CVn as being precise to the 1% level, critical in understanding if this class of binary survives through a period minimum or coalesce. New Hubble Space Telescope ultraviolet imaging and spectroscopic data allowed the isolated detection of the AM CVn at shorter wavelengths, revealing an approximately 58 000 K accretor white dwarf, while placing a firm distance constraint of 3510+140−110 pc. At this distance, we predict that the Laser Interferometer Space Antenna (LISA) will detect V407 Vul with a 28.4 ± 9.2 signal-to-noise ratio in a 4 yr mission time, making it the first verification binary with an outer tertiary, or ‘verification triple’, detectable for millihertz gravitational wave observatories. https://research-explorer.ista.ac.at/download/23238/23263/2026_AstronomyAstrophysics_Munday.pdf application/pdfno https://creativecommons.org/licenses/by/4.0/ EDP Sciences2026 eng 0004-6361 1432-0746 2607.2555710.1051/0004-6361/202661518 714 Munday, James, Antonio C. Rodriguez, Nicole Reindl, Nina Mackensen, Tin Long Sunny Wong, S. P. Littlefair, Ingrid Pelisoli, et al. “V407 Vul: A Triple Star System with an AM CVn Detectable by Gravitational Wave Observatories.” <i>Astronomy &#38; Astrophysics</i>. EDP Sciences, 2026. <a href="https://doi.org/10.1051/0004-6361/202661518">https://doi.org/10.1051/0004-6361/202661518</a>. Munday, James, et al. “V407 Vul: A Triple Star System with an AM CVn Detectable by Gravitational Wave Observatories.” <i>Astronomy &#38; Astrophysics</i>, vol. 714, A200, EDP Sciences, 2026, doi:<a href="https://doi.org/10.1051/0004-6361/202661518">10.1051/0004-6361/202661518</a>. Munday, J., Rodriguez, A. C., Reindl, N., Mackensen, N., Sunny Wong, T. L., Littlefair, S. P., … Yates, A. (2026). V407 Vul: A triple star system with an AM CVn detectable by gravitational wave observatories. <i>Astronomy &#38; Astrophysics</i>. EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/202661518">https://doi.org/10.1051/0004-6361/202661518</a> Munday J, Rodriguez AC, Reindl N, et al. V407 Vul: A triple star system with an AM CVn detectable by gravitational wave observatories. <i>Astronomy &#38; Astrophysics</i>. 2026;714. doi:<a href="https://doi.org/10.1051/0004-6361/202661518">10.1051/0004-6361/202661518</a> J. Munday, A.C. Rodriguez, N. Reindl, N. Mackensen, T.L. Sunny Wong, S.P. Littlefair, I. Pelisoli, A. Brown, N. Castro Segura, J. Chakraborty, H. Dawson, V.S. Dhillon, M. Dorsch, M. Dyer, J.A. Garbutt, S. Geier, M. Green, D. Jarvis, M.R. Kennedy, P. Kerry, J. Mccormac, S.G. Parsons, J.C. van Roestel, D. Sahman, P.E. Tremblay, A. Yates, Astronomy &#38; Astrophysics 714 (2026). J. Munday <i>et al.</i>, “V407 Vul: A triple star system with an AM CVn detectable by gravitational wave observatories,” <i>Astronomy &#38; Astrophysics</i>, vol. 714. EDP Sciences, 2026. Munday J, Rodriguez AC, Reindl N, Mackensen N, Sunny Wong TL, Littlefair SP, Pelisoli I, Brown A, Castro Segura N, Chakraborty J, Dawson H, Dhillon VS, Dorsch M, Dyer M, Garbutt JA, Geier S, Green M, Jarvis D, Kennedy MR, Kerry P, Mccormac J, Parsons SG, van Roestel JC, Sahman D, Tremblay PE, Yates A. 2026. V407 Vul: A triple star system with an AM CVn detectable by gravitational wave observatories. Astronomy &#38; Astrophysics. 714, A200. 232382026-11-08T23:01:57Z2026-11-12T07:51:14Z