The nature of the association

The physical parameters of both stars are presented in Table 4. They were derived from the new calibrations in the Geneva Photometric System (Künzli et al., 1997). An estimation of the masses is obtained utilising the calibrations from Kobi and North (1990) and North (1996). Spectral types are from Gray and Garrison (1989). Rotational velocities are from Levato (1975). Bolometric corrections have been taken from Flower (1996). In addition, as stated in the introduction, the Hipparcos trigonometric parallaxes and proper motions are sufficiently concordant to confirm the classification as a common proper motion pair. Both stars also share the same rotational velocity. We checked for radial velocity data as a further evidence but, regrettably, were not able to find any (e.g. in the Wilson Evans Batten catalogue (Duflot et al., 1995)).

Table 4: Physical parameters for HD 220392/1

Identifiers	HD	220392	220391
	Hip	115510	115506
	CCDM	23239-5349A	23239-5349B
Sp. Type		F0IVn	A9Vn
mv	mag	6.124 $\pm$ 0.014	7.103 $\pm$ 0.007
U	mag	1.608	1.538
V	mag	0.647	0.662
B1	mag	0.958	0.954
B2	mag	1.422	1.426
V1	mag	1.364	1.379
G	mag	1.769	1.790
d	mag	1.314	1.259
B2-V1	mag	0.058	0.047
MV	mag	+0.83 $\pm$ 0.15	+1.62 $\pm$ 0.15
Mbol	mag	+0.87 $\pm$ 0.15	+1.66 $\pm$ 0.15
log Teff	K	3.856 $\pm$ 0.008	3.867 $\pm$ 0.009
$\lbrack M/H]$	dex	-0.05 $\pm$ 0.09	-0.12 $\pm$ 0.10
log g	dex	3.77 $\pm$ 0.07	4.06 $\pm$ 0.07
$\mathcal{M}$	$\mathcal{{M}_{\odot }}$	2.3 $\pm$ 0.2	1.8 $\pm$ 0.2
$vsin{\it i}$	kms-1	165	140

We used the absolute magnitudes derived from the photometric calibration to fit a model of stellar evolution of solar chemical composition (Schaller et al., 1992) in a theoretical H-R diagram. The same isochrone with an estimated age of $\approx$ 10⁹ years for the system appears to fit both stars well, as also verified by Tsvetkov (1993). We conclude that both stars either form a common origin pair or perhaps a true binary system.