COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2258

                                    Konkoly Observatory
                                    Budapest
                                    1983 January 3

                                    HU ISSN 0374-0676



      COORDINATED ULTRAVIOLET, OPTICAL AND RADIO OBSERVATIONS
                 OF RS CVn AND FLARE STARS



 Time has been allocated on the International Ultraviolet Explorer (IUE)
satellite to three groups based at Armagh Observatory (Northern Ireland),
Catania Observatory (Italy) and JILA (Boulder, USA) by the British SERC, ESA
and NASA Agencies, respectively, to observe RS CVn and BY Dra flare stars
during the period 1-7 February 1983. The total time allocated (12 eight-hour
shifts) is comparable to the period of RS CVn and BY Dra-type optical variability 
and many times the mean inter-arrival time between flares. As a result
it is hoped to be able to monitor changes taking place in the star's UV 
spectrum along a complete cycle of spot-like slow variation and during any fast
transient phenomenon, with particular reference to those lines indicative of
physical conditions in the chromosphere, transition region and lower corona.
 A similar coordinated program was carried out successfully in October 1981,
and preliminary important results have been presented at several recent
meetings: ADVANCES IN ULTRAVIOLET ASTRONOMY, Boulder, Colorado, NASA-CP 2238
(January 1982), THIRD EUROPEAN IUE CONFERENCE, Madrid, ESA-SP 176' (May 1982),
ACTIVITY IN RED-DWARF STARS, IAU Colloquium 71, Catania (August 1982). A
complete account of the 1981 collaborative program is in preparation.

 We are planning to observe three RS CVn systems (RS CVn, V 711 Tau =
HR 1099, II Peg = HD 224085) along a complete cycle of variability
and one flare star (AD Leo or YZ CMi). Concurrent photoelectric, spectroscopic
and radio observations of these stars would add considerably to the scientific
program in the UV band.
 Simultaneous observations are not essential for the RS CVn program, as we
are mainly interested in detecting spot/plage modulation of the stellar 
emission vs rotation phase, which is normally a stable phenomenon over a few
cycles. Nevertheless simultaneous coverage is highly desirable, whenever
possible, as useful information can be obtained on any transient phenomenon
that occur during the IUE exposure.



  Of course, simultaneous observations of the selected flare star (YZ CMi
and AD Leo) will be essential, as the study of UV-associated flare events is
the principal aim of the flare star program.
  Therefore we are appealing all photometric, spectroscopic and radio observers 
to cooperate in providing the necessary coverage. Photometric observations
should consist of U, B, V preferably photoelectric measurements of RS CVn
stars with respect to the indicated comparison stars (Table I).


                    Table I


        IUE Program Stars and suggested comparison stars

Gliese  Name  HD/DM     SAO      (1950)           (1950) V(max)  Sp   Period

501.1   RS CVn 114519   063382  13h08m18s+36deg12'01",  7.9  F4V+K0IV  4.7979d
        C 1 +35deg2420  063401
        C 2 +36deg2347  063397
-       V711Tau 22468   111291   033413    +002533    6.0   G5IV+K1IV 2.84
        C 1 22484       111292
        C 2 22796       111334
-       II Peg 224085   091578   235229   +2821 IS    7.3   K2IVe+ ?   6.72
        C 1 +28deg4667  091577
        C 2 +28deg4648  091593
        C 3 224016      091568
388     AD Leo +20deg2465 081292    101654    +200719    9.4   M4.5Ve     -
        C 1 +21deg2175    081296
        C 2 +20deg2460    081274
285     YZ CMi -        -        074204    +034048   11.2   dM4.5e     2.78
        (a) +03deg1778    115869
        (b) -           -              (cf. finding chart by A.D. Andrews,
                                       IBVS 342,1969)

        (e), +04deg1806




Transformation to the standard UBV system is desirable. We urge photometric
observes to start their observations before the actual period of IUE observations 
and, if necessary, to prosecute them in order to secure a good phase
coverage of the light curves. Continuous photoelectric monitoring of the
flare stars in the standard U or similar wave band, with integration times
of between 1 to 10 seconds, during the IUE exposures is recommended. Comparison 
star measurements (cf. Table I) should be carried out before and after
the IUE exposure /photoelectric monitoring of flare stars.




 As soon as available, the final details of the observation schedule will
be communicated directly to those who have shown their interest by contacting
one of the undersigned at the address, or better at the telex indicated below.




         P.H. BYRNE, C.J. BUTLER, A.D. ANDREWS
         Armagh Observatory, Armagh BT61 9 DG
         Northern Ireland
         (TELEX: 747937 ARMOBS G)


         M. RODONO, S. CATALANO, V. PAZZANI
         Osservatorio Astrofisico and University
         Universita di Catania, 95125 Catania
         Italy
         (TELEX: 970359 ASTRCT I)


         J.L. LINSKY, P. BORNMAN
         JILA/NHS, University of Colorado
         Boulder, CO 80309
         U.S.A.
         (TELEX: 45897 SOLTERWARN BDR)


         B.M. HAISCH
         Loockheed Research Laboratory
         Dept. 52-12, Bldg. 255
         3251 Hanover Street, Palo Alto, CA 94304
         U.S.A.
         (TELEX: 8953046 LKHD G)