COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                       Number 2819

                                           Konkoly Observatory
                                           Budapest
                                           15 November 1985

                                           HU ISSN 0374 - 0676




     FLARE MONITORING OF THE ACTIVE LATE-TYPE SYSTEM YY GEMINORUM


  In response to a request by Patrick A. Wayman, Dunsink Observatory, for 
securing simultaneous optical flare monitoring of YY Gem during the period the
object was under observation with EXOSAT and IUE satellites, the variable was
set on the observing schedule of Hoher List Observatory. Unfortunately, 
unfavourable weather conditions did not permit simultaneous observations, and the
only successful observing run, which started on 1984, Nov 12/13 at 23:58 UT,
had to be terminated at 3:42 UT, just before the onset of the IUE observations.
  The instrumentation used was a double beam photometer attached to the 
Nasmyth focus of the 106 cm Cassegrain telescope. The photometer is especially
suited for observations of the light curves of rapid variables and allows the
simultaneous monitoring of the variable and a comparison star within the field
of the telescope (Geyer and Hoffmann, 1975).
  Due to the bright moonlight sky and strong stray light from Castor AB, the
U and B bands yielded a very low S/N ratio, which forced us to do the monitoring
in the V band only. So we could expect only small flare amplitudes, 
nevertheless detectable with the superior performance of a double beam photometer.
The observing strategy was to monitor variable and comparison star (BD+
31deg1611) as continuous as possible, with breaks for casual sky measurements
(every 15 minutes on the average) and centering controls. Measurements were
recorded with a strip chart recorder, time constant was 1 sec.
  The reductions presented some unexpected difficulties. In the raw magnitudes 
minor drift effects (typically about 0.01m or 0.02m), accumulated during
the rather long phases of unattendedness, became apparent and forced us to do
a lot of data editing. Second, there was the problem of light contamination
from Castor AB. Sky readings taken in the vicinity of the variable were on the
the average ten times higher than near the comparison star. Moreover, due to
the strong forward scattering and (presumably) some minor inconsistencies in
making the sky offset, those high sky deflections are more variable than they
ought to be. So we decided to use only the sky deflections taken at the



                                 [FIGURE 1]

 Figure 1. Visual light curve of YY Gem, obtained during 3.5 hours of 
 flare monitoring while secondary eclipse was in progress. Flare events 
 are indicated by arrows. Differential magnitudes, Delta v are given in 
 the sense "YY Gem minus BD +31d 1611".




   Table I.  Characteristics of observed flares of YY Geminorum
        on Nov 12/13, 1984 (23:58 - 03:42 UT).


No.   Start (UT)1    JD hel   Rise time   total duration  Delta m
1     00h36m08s     0.5280   0.4min      ~ 17min          0.09m
2     01 14 28      0.5546   0.1min      ~ 20min          0.09
3     02 09 52      0.5931   0.3min      ~ 20min          0.07
4     03 10 44      0.6354   0.3min        27min          0.15


                              [FIGURE 2]

          Figure 2.  Intensity curves for flare events 1 and 2


comparison, scaled up with an appropriate factor when applied to the variable.
For the subtraction of the sky background, the measured deflections were 
interpolated with splines. Obviously, the variable sky background remains a 
major source of potential systematic errors. For flare monitoring, this should
not be very serious because any effects must show up in both the variable and
the comparison star deflections. Finally, the differential V magnitudes were
evaluated. In order to preserve the internal accuracy of the light curve (the
S/N ratio was better for the variable than for the comparison star), smoothed
comparison star magnitudes were subtracted. Due to the proximity of the 
comparison and the use of the V band no extinction corrections were necessary.
  Figure 1 shows the light curve obtained. The observations cover the whole
secondary minimum, a total phase is indicated. The moment of minimum light,

               Min II = HJD 2446017.5488

was evaluated graphically, using only the undisturbed phase interval from JD
.5425 to .5545. There is evidence for strong light curve distortion, presumably
due to star spot activity; see, e.g., Kron (1952). During 3.5 hours of



observing we noticed four flarelike events whose characteristics are presented 
in Table I. Intensity curves of flare events 1 and 2 (that were recorded
on the descending and ascending branch of the minimum, resp.) are displayed
in Figure 2.
  The individual observations will be deposited in the I.A.U. Archives of
Unpublished Observations of Variable Stars.



                            E. H. GEYER     B.-C. KAMPER
                            Observatorium Hoher List
                          der Universitatssternwarte Bonn

                            D- 5568 Daun/Eifel, F.R.G.



References:


Geyer, E.H., Hoffmann, M.: 1975, Astron. Astrophys. 38, 359. [BIBCODE 1975A&A....38..359G ]
Kron, G.E.: 1952, Astrophys. J. 115, 301. [BIBCODE 1952ApJ...115..301K ]


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