Negative Welch-Stetson Index

[jg <jg@jgws.freeisp.co.uk>]

I was ploughing through +7 degrees on the sky (in Guide) generally
looking for where TASS CD 23 objects with Welch-Stetson values from
using wsv3.pl against collected.big coincided with red stars of various
sorts when I came across object 34449 from collected.big

It had a negative Welch-Stetson index.  Not only that, by happenstance
it had the biggest Welch-Stetson index from that dataset of about -11.

So I decided I'd better read the paper at last, and did so (avoiding as
best I could all lines in it beginning with capital sigmas).

It seems that negative Welch-Stetson indices flag problem multi-epoch
two colour photometric data that can emulate changes due to
variability.  If I've read it right.

34449 has just two nights of observations, and is quite red with V-I
about two and a half.  Unlike the usual case with red stars, both V and
I are a bit scattered, whereas V is usually a bit scattered but I is
quite tight, from a few red stars of this colour I've looked at with the
TASS MkIV data.

Another case is 31388 (WS index -6) where the second of two, distant,
nights' run of data is brighter in I and fainter in V.  I is a bit
scattered here too.

As said, other red stars in the batch are okay, sometimes with quite
large positive WS index (when variable, eg CGCS 1749), so obviously not
an instrumental effect.

Most of the rest of the "bottom ten" WS index objects I scanned via a
graphical plot of their fields, and they appeared to consist of similar
brightness stars within about half an arcmin or less of each other, a
couple of examples apparently being pairs of fairly different colour.

1869 stars of the 94,500+ stars in collected.big from CD 23 gave a
negative WS index, and of those a mere 180 are less than WS index = -1


Cheers

John

John Greaves
UK