COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2200

                                 Konkoly Observatory
                                 Budapest
                                 1982 September 27

                                 HU ISSN 0374-0676




            AN INTENSE OPTICAL FLARE FROM XY UMa




   As part of our on-going program of UBVR photometry of RS CVn stars, we
have observed at four colors a flare episode from XY UMa (+55d 1317, SAO
27143), a system with a 0.48 day period. The primary star is G2-G5; the 
secondary K5 (Geyer, quoted by Loronzi and Scaltriti, 1977). All observations
were made with the Capilla Peak Observatory's 61-cm reflecting telescope and
a single-channel, photon-counting photometer with an EMR 641A tube cooled to
-20dC. The comparison star was +54d 1278 (SAO 21739).
   The flare occurred on 31 January 1982 UT as the system was emerging from
secondary eclipse at about phase 0.54, the time was 05:14 UT. Figures 1-4
show the flare in the UBVR instrumental bands relative to the underlying
light curve. The symbol "+" indicates actual data points; the solid dots are
a Stineman fit to the data; the solid lines the average light curve of the
system with the flare removed. The data points have a statistical accuracy of
+-0.01 mag or better.
   The flare peak occurred at about phase 0.55 at 05:29 UT. The total duration 
of the main body of the flare was about 30 minutes. Note that a much
less intense tail appears after the main burst [this tail is especially visible
at the V and R bands). Relative to the average light curve of the system, the
peak increase in apparent magnitude was 0.30 at U, 0.13 at B, 0.09 at V, and
0.04 at R. At peak, V=9.62, R=8.86, B-V=+0.77, and U-B=0.27.
   No optical flares have been previously found in photometric or polarimetric 
(Geyer and Metz, 1977) observations of XY UMa. However, Patkos (1981)
found three flares in 1980 from SV Cam, a system that resembles XY UMa (primary
G3, secondary K4, period 0.59 d). The strongest of the three was seen at phase
0.61 as the system emerged from secondary eclipse; its duration was 43 min.
and peaked at 0.12 mag above the system at U. Our XY UMa flare had a similar
duration but a much more intense peak and total flux.

                              [FIGURE 1]
                              [FIGURE 2]
                              [FIGURE 3]
                              [FIGURE 4]

The integrated fluxes are:

Filter         Flux          Flare/Base
  U        2.5 x 10^4 mjy*sec   0.10
  B        4.6 x 10^4           0.05
  V        8.0 x 10^4           0.05

where the last column gives the ratio of the flare's flux to the underlying
flux of the system over the same period of time.


             M. ZEILIK, R. ELSTON, and G. HENSON

                 Capilla Peak Observatory
               The University of New Mexico
                  Albuquerque, NM 87131
                          U.S.A.





References:

Geyer, E. H., Astrophys. and Space. Sci., Vol. 48, 137, 1977. [BIBCODE 1977Ap&SS..48..137G ]
Geyer, E. H. and Metz, K., Astrophys. and Space Sci., Vol. 52, 35, 1977. [BIBCODE 1977Ap&SS..52..351G ]
Lorenzi, L. and Scaltriti, F., Acta Astron., Vol. 27, 273, 1977. [BIBCODE 1977AcA....27..273L ]
Patkos, L., Astrophys. Lett., Vol. 22, 131, 1981. [BIBCODE 1981ApL....22..131P ]