April 30, 2026 report
GP Com observations sharpen picture of a rare ultracompact binary system
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
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Using the Rozhen National Astronomical Observatory, Bulgarian astronomers have conducted optical photometric observations of an ultracompact binary known as GP Com. Results of the observational campaign, presented in the Proceedings of the Bulgarian Academy of Sciences, shed more light on the properties of this system.
Flickering white dwarfs eating their companions
Cataclysmic variables (CVs) are close binary star systems consisting of a white dwarf (WD) primary that is accreting matter from (usually) a main sequence star. They irregularly increase in brightness, by a large factor, then drop back down to a quiescent state.
AM Canum Venaticorum (AM CVn) systems are a rare type of CV named after AM Canum Venaticorum—a hydrogen-deficient cataclysmic variable binary in the constellation of Canes Venatici.
In general, AM CVn-type systems are ultracompact, mass-transferring binaries with orbital periods between five and 68 minutes. They consist of a white dwarf accreting helium-dominated matter from a degenerate or semi-degenerate donor star. To date, only about 120 AM CVns have been discovered.
A rare CV just 237 light years away
Located some 237 light years away from Earth, GP Com is one of such rare AM CVn systems. It has an orbital period of approximately 46.57 minutes and is one of the brightest AM CVns known. The system is composed of a white dwarf primary and a very low-mass, helium-rich object.
Given that very little is known regarding the properties of GP Com, a team of astronomers led by Radoslav Zamanov of the Bulgarian Academy of Sciences decided to explore this system with the 2m RCC telescope and with the 1.5m AZ1500 telescope of the Rozhen National Astronomical Observatory in Bulgaria.
"We report 10.6 hours of photometric observations of the ultrashort period binary GP Com, consisting of a white dwarf and the helium core of a former main sequence star. The observations are obtained with the telescopes of Rozhen National Astronomical Observatory," the researchers write in the paper.
Digging into the nature of GP Com
The observations found that GP Com has an orbital modulation with amplitude 0.04−0.05 mag in B and V bands. The system reaches maximum brightness of 16.04 and 16.05 mag in B and V bands, respectively.
Previous studies of GP Com have revealed the presence of a strong bright spot near the expected accretion stream/disk impact region. The new observations performed by Zamanov's team found that this bright spot has a temperature of about 19,700 K.
Furthermore, the observations found that GP Com has a mass accretion rate at a level of 2×10-12 solar masses per year. This is a few times lower than previously estimated and is consistent with the predicted mass accretion rate for a cool donor star.
The results of the new study also put some constraints on the properties of the donor star in GP Com. The collected data suggest that the donor has a radius of about 0.054 solar radii and a mass of around 0.025 solar masses, which indicates that it is a helium star rather than a white dwarf.
Written for you by our author Tomasz Nowakowski, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.
More information
Radoslav Zamanov et al., BV Photometry of the Ultracompact Binary Star GP Com, Proceedings of the Bulgarian Academy of Sciences (2026). DOI: 10.7546/CRABS.2026.03.01
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