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Antoine Couëtte wrote:
> If I remember correctly - and only if ... :-) - , R Kent is a double (if not 
a triple or quadruple) Star with (main ?) Companion Star quite bright 
currently seen at a rather big angular distance from Main Star, the latter 
also being its brightest Body.
>
> In the Former FK4 Stars Catalog, they published its Gravity Center position 
with a correction from GC to the Main Star. That's the way they tackled its 
peculiarities.

Alpha Centauri is a triple system. Stars A and B are approximately mag.
0 and 1, and definitely a true pair. The maximum separation is .3 or .4
arc minutes and the orbit period about 80 years. I think the star
Proxima Centauri is not universally accepted as a true system member.
It's much too dim to see in a sextant, so we can ignore it.

For a celestial navigation almanac I would use the position and proper
motion of the system's center of mass. Because the stars have similar
mass and luminosity, we can assume the center of mass and center of
light are practically the same. This point will have constant velocity
through space, though the A and B stars individually do not.

Via the CDS VizieR system I found alpha Centauri in the FK4 catalog as
number 538. It's also in the FK5 under the same number. (Maybe FK6 too -
I didn't check.) However, I cannot tell if the catalog data are for an
actual star, or the system's center of gravity.


http://vizier.cfa.harvard.edu/viz-bin/VizieR-5?-ref=VIZ521d751a3501&-out.add=.&-source=I/143/fk4_1950&recno=510

The Hipparcos catalog gives the proper motions of A and B as if they
were each moving in a straight line. But it's been more than 20 years
since the epoch of the catalog (1991.25). That's 1/4 of an orbit. Also,
the catalog notes say the solution for these two stars was indeterminate.

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