CM Draconis

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Coordinates: Sky map 16h 34m 20.321s, +57° 09′ 44.70″

CM Draconis
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Draco
Right ascension 16h 34m 20.321s
Declination +57° 09′ 44.70″
Apparent magnitude (V) 12.90
Spectral type M4.5V / M4.5V / DB
Variable type BY Draconis variable
Eclipsing binary
Flare star
Radial velocity (Rv) –118.71 km/s
Proper motion (μ) RA: –1109 mas/yr
Dec.: 1203 mas/yr
Parallax (π) 69.2 ± 2.5[1] mas
Distance 47 ± 2 ly
(14.5 ± 0.5 pc)
Absolute magnitude (MV) 12.1
Other designations
Database references

CM Draconis (GJ 630.1A) is an eclipsing binary system approximately 47 light-years away[1] in the constellation of Draco (the Dragon). The system consists of two nearly identical red dwarf stars located in the constellation Draco. The two stars orbit each other with a period of 1.27 days with a separation of 2.7 million kilometres.[2] Along with two stars in the triple system KOI 126, the stars in CM Draconis are the lightest stars with precisely measured masses and radii. Consequently, the system plays an important role in testing stellar structure models for very low mass stars.[3] These comparisons find that models underpredict the stellar radii by approximately 5%. This is attributed to consequences of the stars' strong magnetic activity. According to the system's entry in the Combined General Catalogue of Variable Stars, at least one of the components is a flare star and at least one is a BY Draconis variable.[4] The white dwarf star GJ 630.1B, located 25.7 arcseconds away shares the same proper motion as the CM Draconis stars and is thus a true companion star of the system.[5] Given the system's distance of 47 light years,[6] this corresponds to a separation of at least 370 astronomical units between CM Draconis and GJ 630.1B.


CM Draconis distance estimates

Source Parallax, mas Distance, pc Distance, ly Ref.
Gliese & Jahreiß (1991) 69.0 ± 2.9 14.5 ± 0.6 47.3 ± 2.0 [6]
van Altena et al. (1995) 69.2 ± 2.5 14.5 ± 0.5 47.1 ± 1.8 [1]

The most precise estimate is marked in bold.

Possible planetary system[edit]

The system was the subject of a dedicated search for transiting extrasolar planets in orbit around the binary from 1994–1999. In the end, the existence of all of the transiting planet candidates suggested by the project was ruled out.[7][8]

Based on variations in the timing of the system's eclipses, it has been suggested that there may be an object in a circumbinary orbit around the two red dwarf stars. In 2000, it was proposed that a Jovian planet is orbiting the system with a period of 750–1050 days.[9] More recent eclipse measurements suggest there may be a planet with a mass similar to Jupiter in an 18.5-year orbit, or instead a more massive object located further out. The variations are inconsistent with being caused by the white dwarf companion star. No further evidence for the 750–1050 day planet was found and it is considered unlikely that it exists.[5]

The CM Draconis system[5]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b (unconfirmed) ≥1.5 MJ 5.3 18.5±4.5  ?


  1. ^ a b c d van Altena et al. (1995)[full citation needed]
  2. ^ "TEP: CM Draconis". 
  3. ^ Morales, Juan Carlos; et al. (2009). "Absolute Properties of the Low-Mass Eclipsing Binary CM Draconis". Astrophysical Journal 691 (2): 1400. arXiv:0810.1541. doi:10.1088/0004-637X/691/2/1400. 
  4. ^ Samus (2004). "CM Dra". Combined General Catalogue of Variable Stars. 
  5. ^ a b c Deeg, H. J.; Ocaña, B.; Kozhevnikov, V. P.; Charbonneau, D.; O'Donovan, F. T.; Doyle, L. R. (2008). "Extrasolar planet detection by binary stellar eclipse timing: evidence for a third body around CM Draconis". Astronomy and Astrophysics 480 (2): 563–571. arXiv:0801.2186. Bibcode:2008A&A...480..563D. doi:10.1051/0004-6361:20079000. 
  6. ^ a b Gliese, W. and Jahreiß, H. (1991). "GJ 630.1A". Preliminary Version of the Third Catalogue of Nearby Stars. Retrieved 2008-10-12. 
  7. ^ "The TEP network". 
  8. ^ Doyle, Laurance R.; Deeg, Hans J.; Kozhevnikov, Valerij P.; Oetiker, Brian; Martín, Eduardo L.; Blue, J. Ellen; Rottler, Lee; Stone, Remington P. S.; Ninkov, Zoran; Jenkins, Jon M.; Schneider, Jean; Dunham, Edward W.; Doyle, Moira F.; Paleologou, Efthimious (2000). "Observational Limits on Terrestrial-sized Inner Planets around the CM Draconis System Using the Photometric Transit Method with a Matched-Filter Algorithm". The Astrophysical Journal 535 (1): 338–349. arXiv:astro-ph/0001177. Bibcode:2000ApJ...535..338D. doi:10.1086/308830. 
  9. ^ Deeg, H. J.; Doyle, L. R.; Kozhevnikov, V. P.; Blue, J. E.; Martín, E. L.; Schneider, J. (2000). "A search for Jovian-mass planets around CM Draconis using eclipse minima timing.". Astronomy and Astrophysics 358: L5–L8. arXiv:astro-ph/0003391. Bibcode:2000A&A...358L...5D. 

External links[edit]