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HD 42936
	; Location of HD 42936 in the night sky. The star is marked within the red diamond.
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Mensa
Right ascension	06h 06m 29.84725s
Declination	−72° 30′ 45.5674″
Apparent magnitude (V)	9.10
Characteristics
HD 42936A
Evolutionary stage	main sequence star
Spectral type	K0 IV/V
B−V color index	+0.91
HD 42936B
Evolutionary stage	main sequence star
Spectral type	L
Astrometry
Radial velocity (Rv)	35.64±0.49 km/s
Proper motion (μ)	RA: −0.716 mas/yr ; Dec.: −382.755 mas/yr
Parallax (π)	21.2496 ± 0.1143 mas
Distance	153.5 ± 0.8 ly ; (47.1 ± 0.3 pc)
Absolute magnitude (MV)	5.78
Orbit
Primary	HD 42936A
Companion	HD 42936B
Period (P)	506.89±0.01 d
Semi-major axis (a)	1.139±0.004 AU
Eccentricity (e)	0.596±0.001
Inclination (i)	63.89±0.78°
Argument of periastron (ω); (secondary)	158.88+0.03; −0.01°
Semi-amplitude (K1); (primary)	2.65731+0.00033; −0.00002 km/s
Details
HD 42936A
Mass	0.900±0.009 M☉
Radius	0.861±0.005 R☉
Luminosity	0.510±0.003 L☉
Surface gravity (log g)	4.266±0.045 cgs
Temperature	5,201±20 K
Metallicity [Fe/H]	0.147±0.013 dex
Rotation	21.8+0.5; −16.5 d
Rotational velocity (v sin i)	3.17±0.1 km/s
Age	9.6±0.8 Gyr
HD 42936B
Mass	91.90±0.85 MJup
Other designations
	DMPP-3, CD−72°312, CPD−72°451, GC 7845, HD 42936, HIP 28941, SAO 256269
Database references
SIMBAD	data
Exoplanet Archive	data

HD 42936, also known as DMPP-3, is a star located in the southern circumpolar constellation Mensa. With an apparent magnitude of 9.1,^[2] it is too faint to be detected with the naked eye but can be seen with a telescope. The star is relatively close at a distance of about 153 light-years (47 parsecs) but is receding with a heliocentric radial velocity of 35.6 km/s.^[1]

HD 42936 is an early K-type star with the blended luminosity class of a main sequence star and a subgiant.^[3] At present it has 87% the mass of the Sun and 91% the radius of the Sun.^[8] The object shines at 51% the luminosity of the Sun^[7] from its photosphere at an effective temperature of 5,138 K,^[8] which gives it an orangish yellow glow. HD 42936 has iron abundance 151% that of the Sun, meaning it is metal enriched^[10] despite an age of 10.9 billion years.^[8]

HD 42936 has a very low mass companion star in a close orbit,^[8] approaching to 0.498 AU at periastron.^[6]

Planetary system

In 2019, a radial velocity analysis carried out by a team of astronomers led by astronomer John R. Barnes of the Dispersed Matter Planet Project (DMPP) confirmed the existence of a super-Earth in orbit around DMPP-3 A. Planets in close binary star systems such as this are rare.^[8]

A follow-up study in 2023 refined the parameters of the planet and companion star, and detected two additional radial velocity signals. One of these could be caused by a second, Earth-mass planet closer to the star, but the other, 800-day signal cannot be caused by an orbiting body because the companion star would make its orbit unstable. The study concludes that the 800-day signal must be caused by stellar activity, but if not for the companion star it could have been considered a likely planet, which has implications for other radial velocity planet detections.^[6]

The DMPP-3 A planetary system^[6]
Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
c (unconfirmed)	≥1.065+0.173 −0.259 M_🜨	0.033+0.002 −0.0001	2.26+0.20 −0.10	0	—	—
b	≥2.22+0.50 −0.28 $M$ _🜨	0.0670+0.0003 −0.0002	6.6732+0.0011 −0.0003^[8]	0.174+0.032 −0.084	—	—

References

^ ^a ^b ^c ^d ^e ^f Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
^ ^a ^b ^c Anderson, E.; Francis, Ch. (May 2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331–346. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. ISSN 1063-7737. S2CID 255204555.
^ ^a ^b Houk, N.; Cowley, A. P. (1975). University of Michigan Catalogue of two-dimensional spectral types for the HD stars. Volume I. Declinations -90_ to -53_ƒ0. Bibcode:1975mcts.book.....H.
^ Cousins, A. W. J.; Stoy, R. H. (1962). "Photoelectric magnitudes and colours of Southern stars". Royal Greenwich Observatory Bulletins. 64: 103. Bibcode:1962RGOB...64..103C.
^ Haswell, Carole A.; Staab, Daniel; Barnes, John R.; Anglada-Escudé, Guillem; Fossati, Luca; Jenkins, James S.; Norton, Andrew J.; Doherty, James P. J.; Cooper, Joseph (2019). "Dispersed Matter Planet Project discoveries of ablating planets orbiting nearby bright stars". Nature Astronomy. 4 (4): 408–418. arXiv:1912.10874. doi:10.1038/s41550-019-0973-y. S2CID 209444484.
^ ^a ^b ^c ^d ^e Stevenson, Adam T.; Haswell, Carole A.; et al. (May 2023). "DMPP-3: confirmation of short-period S-type planet(s) in a compact eccentric binary star system, and warnings about long-period RV planet detections". Monthly Notices of the Royal Astronomical Society. arXiv:2305.06263.
^ ^a ^b Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ ^a ^b ^c ^d ^e ^f ^g Barnes, John R.; Haswell, Carole A.; Staab, Daniel; Anglada-Escudé, Guillem; Fossati, Luca; Doherty, James P. J.; Cooper, Joseph; Jenkins, James S.; Díaz, Matías R.; Soto, Maritza G.; Peña Rojas, Pablo A. (2019). "An ablating 2.6-M_🜨 planet in an eccentric binary from the Dispersed Matter Planet Project". Nature Astronomy. 4 (4): 419–426. arXiv:1912.10793. doi:10.1038/s41550-019-0972-z. S2CID 209444780.
^ HD 42936, entry, SIMBAD. Accessed online December 24, 2019.
^ Jenkins, J. S.; Jones, H. R. A.; Pavlenko, Y.; Pinfield, D. J.; Barnes, J. R.; Lyubchik, Y. (July 2008). "Metallicities and activities of southern stars". Astronomy & Astrophysics. 485 (2): 571–584. arXiv:0804.1128. Bibcode:2008A&A...485..571J. doi:10.1051/0004-6361:20078611. ISSN 0004-6361.