V838 Herculis declined from its peak brightness very quickly, fading by 2 magnitudes in less than three days, making it one of the fasted classical novae ever recorded.
All novae are binary stars, with a "donor" star orbiting a white dwarf. The two stars are so close to each other that material is transferred from the donor to the white dwarf. Because the distance between the two stars is comparable to the radius of the donor star, novae are often eclipsing binaries, and V838 Herculis does show such eclipses. The eclipses were first detected a few weeks after the nova outburst, and they show the system's orbital period to be 7 hours, 8 minutes and 36 seconds as of 1991. The shape of the eclipse light curve suggests that the white dwarf itself is not being eclipsed by the donor, but rather that the accretion disk surrounding the white dwarf is being partially eclipsed. The depth of the eclipses was initially only 0.1 magnitudes, but grew over the year following the nova event to 0.7 magnitudes, indicating that the accretion disk re-established itself after the nova outburst during that time.
The white dwarf in the V838 Herculis system is an oxygen-neon-magnesium white dwarf, with a mass of about 1.35, M☉ which is near the Chandrasekhar limit for white dwarf masses. The donor star is believed to be a main sequence star.
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^ abLeibowitz, Elia M.; Mendelson, Haim; Mashal, Ezra; Prialnik, Dina; Seitter, Waltraut C. (February 1992). "The Presence of an Accretion Disk in the Eclipsing Binary System Nova Herculis 1991 Three Weeks after Outburst". Astrophysical Journal Letters. 385: L49. doi:10.1086/186275. Retrieved 3 January 2021.
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^ abIngram, Doug; Garnavich, Peter; Green, Paul; Szkody, Paula (June 1992). "Photometry and Spectroscopy of Nova Herculis 1991". Publications of the Astronomical Society of the Pacific. 104: 402. doi:10.1086/133012. Retrieved 3 January 2021.
^Matheson, Thomas; Filippenko, Alexei V.; Ho, Luis C. (November 1993). "Nova Herculis 1991: Thermonuclear Runaway on a Massive ONeMg White Dwarf". Astrophysical Journal Letters. 418: L29. doi:10.1086/187108. Retrieved 3 January 2021.
^Starrfield, S.; Iliadis, C.; Hix, W.R. (May 2016). "The Thermonuclear Runaway and the Classical Nova Outburst". Publications of the Astronomical Society of the Pacific. 128 (963). arXiv:1605.04294. doi:10.1088/1538-3873/128/963/051001. Retrieved 3 January 2021.
^Hachisu, Izumi; Kato, Mariko (July 2018). "A Light Curve Analysis of Recurrent and Very Fast Novae in Our Galaxy, Magellanic Clouds, and M31". The Astrophysical Journal Supplement Series. 237 (1). arXiv:1805.09932. doi:10.3847/1538-4365/aac833. Retrieved 3 January 2021.
^Hachisu, Izumi; Kato, Mariko (June 2019). "A Light-curve Analysis of 32 Recent Galactic Novae: Distances and White Dwarf Masses" (PDF). The Astrophysical Journal Supplement Series. 242 (2). arXiv:1905.10655. doi:10.3847/1538-4365/ab1b43. Retrieved 3 January 2021.
https://web.archive.org/web/20050915104557/http://www.tsm.toyama.toyama.jp/curators/aroom/var/nova/1990.htm (the page is currently offline)
V838 Herculis at the American Association of Variable Star Observers website