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Notes concerning the observations made by UKIRT for the YY Gem programme in
March 1988.

UKIRT was scheduled to observe on four half nights during which two primary and
two secondary eclipses of YY Gem were to occur. Continuous monitoring in the
K-band simultaneously with V or B was made possible with a dichroic filter and
VISPHOT, a photometer set up to monitor the optical beam. A nodding secondary
mirror provided rapid and repeatable background correction. As spot modulation
effects are relatively more prominent in V and flare effects in B, it was
decided to monitor in K and V during eclipses and in K and B at out of eclipse
phases. Useful coverage of the out-of-eclipse phases by UKIRT turned out to be
quite limited, however. The auto-guider was not functioning at the time and
there were several computer crashes due to the operating system writing data
over its own software. Apparently nobody had used the system with this
configuration before and there was insufficient data storage area to accommodate
the large amount of time resolved data we were accumulating. The computer
crashes caused us to lose all of the electronically stored data with the result
that all the data had to be typed in from the printed output, which fortunately
was complete.

The data were corrected for extinction using the following extinction
coefficients: B 0.27, V 0.10, K 0.07, J 0.10 . Colour equations were derived
from observations made of the following local standards:

BD 32 degs 1577, BD 31 degs 1611, BD 31 degs 1627

and the following scattered standards:

Gliese 699 - Barnard's Star (see note); HD 75138; Gl 299; HD 105601; Gl 347A;
BS 4550; HD 84916; HD 84971; HD 85844; HD 85990; HD 86135; HD 86279; HD 90485

Barnard's star was observed on 4 March 1988 and noticed to be declining rapidly
in B, though stable to 0.01 magnitudes in K. Over the period of observation from
UT 06:50:03 to 06:53:00 it dropped by 0.5 magnitudes in the Johnson B-band. A
comment "flare ?" in the margin by C. J. Butler noted that it was probably the
declining stage of a flare. Though flares are common on dM stars it was rather
unexpected on a star of such late M type.

The following colour equations were derived for the UKIRT optical data using
VISPHOT.

V = Mv - Zv - 0.135 (B-V)
B = Mb - Zb + 0.098 (B-V)

where Mv = 20 - 2.5 log CR(V) - 0.15 A
Mb = 20 - 2.5 log CR(B) - 0.27 A

where CR(V) and CR(B) are the count rates in V and B respectively, and A the
airmass.

Zero Points for individual nights were as follows:

date Zv Zb Zk

3/3/88 2.15 10.68
4/3/88 2.60 2.18 10.68
5/3/88 2.60 2.16 10.73
6/3/88 2.57 2.13 10.68



Data files

The files: UKIRTB_HCJD.DAT, UKIRTV_HCJD.DAT, UKIRTK_HCJD.DAT contain the UKIRT
B, V and K photometric data respectively with HJD-2447000 in the first column
and the magnitude in the second.