June 2016 - Double Star of the Month

16/17 Dra (16 36 13.72 +52 55 27.8) is a 90" binocular pair to be found about 15 degrees preceding the head of Draco and the stars are of magnitude 5.4 and 5.5. William Herschel noted that A itself was a rather unequal but easy pair 4" apart, and he included it is his 1782 catalogue as H I 4. He reported that It is the star to which a line drawn from nu through mu points, at nearly the same distance from mu as mu from nu. The brighter star was recorded as white whilst the fainter was 'white inclining to red'. In mid-2015 the writer found the position angle 106 degs and the separation 2".9. The angular motion is but 8 degrees and the stars have closed up from 4" at discovery. All three components appear to be physically connected.

COO 197 (16 25 17.59 - 49 08 52.2) is a rather faint pair (mags 8.1 and 8.2) in southern Norma, near the border with TrA, which appeared in the catalogues of stars compiled at Cordoba Observatory.

It was first measured as double by R. P. Sellors at Sydney in 1895 using the 11.5-inch refractor. It was found to be in slow direct orbital motion and in 1977 an orbit was calculated with a period of 311 years. In 2008 the writer observed it with the large refractor in Johannesburg and at that time the observed position angle and the calculated value differed by more than 20 degrees. Andreas Alzner then performed a re-calculation of the elements of apparent orbit and found that the period was much longer (1132 years). In mid-2016 the stars will be found at 93 degs and 2".3.

In recent years, observations of this system with high resolution techniques and large telescopes have revealed that it is quite a complex multiple star. Using the NACO infra-red camera on one of the 8.2-metre VLT telescopes in 2004, Chauvin and colleagues found that B was again double at a distance of 0".1. The image in the journal shows the two stars clearly separated, but nearby star A appeared single. However, in 2014 Andrei Tokovinin also resolved A into two unequally bright stars separated by 0".1. It appears likely that there is also a spectroscopic sub-system in either Aa or Ab but which of the stars it can be pinned to is not yet determined. A 12th mag star now at 77 degs and 20" is being left behind.

Bob Argyle - Double Star Section Director