September 2018 - Double Star of the Month

STF 2735 (20 55 40.64 +04 31 57.70) is a pretty pair discovered by William Herschel in 1782. It is 1 degree WNW of 1 Equulei which is a bright triple, although the primary pair is now beyond all but the ground-based arrays.

F. G. W. Struve noted the stars were yellow and ash, whilst Smyth in the Bedford Catalogue noted orange tint and purple. More recently John Nanson on the Star-Splitters blog, using a 5-inch f/15 refractor at x191 thought the primary was white with a weak but noticeable gold-yellow tinge.

The stars form a very long period binary and have moved only 8 degrees in almost 200 years. Gaia DR2 puts them at a distance of 351 light-years. Webb noted that the stars were magnitudes 6.2 and 7.5 but the Washington Double Star Catalog (WDS) reduces the difference in magnitude to 1.0. John Nanson suspected that they were more unequal than that and indeed Gaia DR2 gives G magnitudes (similar to V) of 6.1 and 7.4. Using the Cambridge 8-inch refractor the writer found 283.6 degrees and 2".21 in 2011.

Three degrees west of the nearby dwarf Epsilon Indi is a coarse binocular triple which seems to have evaded Dunlop and been first pointed out by W. S. Jacob and which sits in the WDS catalogue as JC 25 (21 43 59.16 -57 19 30.4).

The two brightest stars sit 152" apart in PA 4 degrees and have both very similar and quite significant annual proper motions (115 mas in RA, -53 mas in Dec) and parallaxes (22.54 mas and 22.49 mas, respectively for A and B) such that they are almost certainly physical. The third star, C, is 187" away in PA 214, is magnitude 7.5 and is unconnected. Gaia DR2 puts it 600 light-years away.

Bob Argyle - Double Star Section Director