emamajek on 13-Sep-2026 05:13:05 GMT about HD 218687
Based on Gaia DR2 astrometry, Fuhrmann & Chini (2018,
2018RNAAS...2...56F
) report 'HD 218687—quadruple. C (Gaia DR2 2815480671757024512) at ρ = 35.6[arcmin].'
WDS J23100+1426 = STF 2986
A : HD 218687 = WDS J23100+1426A (SB1, P=3.632d Griffin 2001 Obs 121, 221)
plx=39.7856+-0.0397 mas, pm = -121.596 -85.371 (+-0.043, 0.033) (GaiaDR3)
Griffin (2001,
2001Obs...121..221G
) reports A is a spectroscopic binary
with Porb=3.631626+-0.000010 day, K=22.17+-0.09km/s, e=0.
B : HD 218687B = LSPM J2309+1425 = WDS J23100+1426B (sep 31.3" from A)
C : Gaia DR2 2815480671757024512 = LSPM J2312+1414 (dubbed C by Fuhrmann & Chini 2018)
Comments on A & B:
For components A and B of WDS J23100+1426 = STF 2986, WDS currently notes "Significantly different proper motion and parallax for components, so non physical", citing Jao+2017 (2017AJ....154..191J). Jao+2017 states "The Washington Double Star Catalog lists the two stars as a visual binary (WDS 23100+1426AB) with a separation of 31".7 (Høg et al. 2000). Both stars have parallaxes and proper motions in Gaia DR1, given in Table 3. As with TYC 3663-371-1, the distances and proper motions given in Table 3 do not support that the two components are physically bound. Their parallaxes differ by ∼4σ and the proper motions in Gaia DR1 do not match. Hence, these two stars are likely not associated." Kervella+2022's analysis using a combination of revised Hipparcos (for A) and Gaia DR3 parallaxes, proper motions, and radial velocities are in good agreement (which is readily apparent from the Gaia DR3 astrometry). The Gaia DR3 parallaxes of A and B differ by only 0.68σ. The difference in tangential velocities between A and B is only 1.33+-0.08 km/s. Further, Griffin 2001 comments on the differences in mean radial velocities between HD 218687 A and B (-1.30+-0.27km/s) "a small enough difference to be entirely compatible with the star' belonging to a gravitationally bound system."
What orbital velocity would one predict for Aa+Ab and B? For adopted masses for Aa, Ab, and B of 1.07, ~0.5[guess], 0.63, and Mtot(AB)~2.2 Msun, and adopting an estimate of the semi-major axis as the current projected A-B separation (779 au), one would predict orbital velocity for A and B of 1.6 km/s. This is remarkably similar to the quadrature sum of Kervella+2022's Vtan anomaly and Griffin 2001's difference in RV for A and B : sqrt(1.33^2 + 1.30^2) ~ 1.86+-0.28 km/s. So the tangential and radial velocities of A and B are consistent with predicted orbital motion, and constituting a physical triple.
Comments on C:
Fuhrmann & Chini (2018,
2018RNAAS...2...56F
) dubbed Gaia DR2 2815480671757024512 as component C to the HD 218687 multiple system, and noted that it is 35.6 arcmin away; however it is not in WDS as of 9/12/2026. Kervella+2022 notes that A is 2136.85" away (53700 au) and B is 2166.60" away (54500 au). They estimate the Vtan anomalies of C vs A and C vs B to be 0.62 km/s and 0.81 km/s respectively, compared to predicted escape velocities of 0.19 km/s and 0.16 km/s, respectively for A and B, and a gravitationally bound score of 0 to both instances. However, it is worth noting that 1) A is a spectroscopic binary, and 2) A and B are orbiting each other -- and these scores to do not include the extra mass of the companion to A, nor the orbital motion of A and B (~1.86+-0.28 km/s, as previously calculated). Hence the differences in Vtan between C and either A or B (0.62, 0.81 km/s) could easily due to the orbital motion of A and B. So the boundedness of C to A+B should not be based on motions that ignore the binarity of A and the orbital motion of Aa+Ab and B.
Using the Gaia DR3 astrometry for A (HD 218687, Gaia DR3 2815542965962559488) and C (Gaia DR2 2815480671757024512), I estimate that the PA for C as measured from A is PA = 108.140520 +-0.000001 deg, separation 2136.848540 +-0.000052 arcsec. The differences in proper motions are del(pmRA), del(pmDec) = -1.50+-0.05, -4.95+-0.04 mas/yr, leading to a Vtan anomaly of 0.616+-0.008 km/s. The 3D separation between A and C is ~108 kau or 0.52 pc. Adopting M(Aa/Ab/B) ~ 2.2 Msun, one would estimate the Jacobi radius (M_J = 1.35 * Mtot^(1/3); Mamajek+2013) of the AB subsystem to be ~1.8 pc, so C could be on a bound orbit to A and B if its velocity with respect to the barycenter of Aa/Ab/B is sufficiently low (v_esc ~ 0.2 km/s).
Hence, the HD 218687 appears to be at least a quadruple (as mentioned by Fuhrmann & Chini 2017), with components Aa+Ab+B+C.
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