COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 2752

                                      Konkoly Observatory
                                      Budapest
                                      24 June 1985

                                      HU ISSN 0374 - 0676


       PHOTOELECTRIC OBSERVATIONS OF THE FLARE STAR AD Leo

 The monitoring of AD Leo was carried out on March 27/28 1984 from 00h51m
to 03h29m UT at La Palma Observatory (I.A.C.) on the Swedish 60 cm
Cassegrain telescope with a single-channel photon counting photoelectric
uvbybeta photometer. This was done within the framework of the Coordinated
Program on Ultraviolet, Optical and Radio Observations of RS CVn and Flare
Stars in March 1984 (Rodono 1983).
  A complete set of measurements in u,Hbetan,Hbetaw filters were obtained with 15
sec resolution time.
  During the monitoring time (Table I) a flare has been recorded. Its
characteristics (Andrews et al. 1969) are given in Table II.

        a.- UT at flare maximum.
        b.- Flare duration ta, tb.
        c.-Flow of the outbursts If/I0, where If is the total intensity
           of the star plus flare minus sky background and I0 is the
           quiet state intensity of the star minus sky background.
        d.- Amplitude of the flare in instrumental magnitude Delta m.
        e.- Standard deviation of random noise fluctuations in magnitude
           sigma=2.5log(I0+sigma)/I0, during the quiet state phase
           immediately preceding the beginning of the flare.
         f.- Air mass at the flare maximum.

  The flare is one of the best ever recorded, with a complete coverage from
Radio to Ultraviolet wavelengths (Rodono et al.1984).

                       Table I

   Date     Monitoring Interval UT     Total Monitoring Time
                                         
   March     00h51m31s                     01h42m36s

    27/28    01 47 53 02 08 49
             02 29 00 03 02 35
             03 12 36 03 29 59




                             Table II
Filter   UTmax    ta tb   Delta t   If/I0   Delta m  sigma(m) Air mass
         
u        03h23m   6m 30s   7m     9.8  2.6   0.47 1.44
Hbetan    "        _  "     -     0.9  0.7   0.23   "
Hbetaw    "        -  "     -     0.6  0.5   0.14   "

  From Figure 1 (lower panel.), we can see a typical light curve with type
I flare characteristic of UV Cet stars, with Delta u of 2-3 magnitude. 
Following
the distribution function of amplitudes (Gurzadyan 1980) it corresponds to
the highest amplitude recorded for AD Leo, with a frequency of 10%.

  The relative flux measurements through Hbetan and Hbetaw filters show peak fluxes
of 0.9 and 0.6 in good agreement with the Hgamma and continuum (4400 angstrom) behaviour
during flare maximum reported by Rodono et al. (1984).


                             [FIGURE 1]




 A noticeable increase of the scatter in the u filter in coincidence with
a Hbetaw deep a few minutes before the maximum could be inferred from the data
shown in Figure 1. A faster flux decrease below the normalization value
in Hbeta filters after the peak should be explained.
  The emission line index Hbeta is formed by taking the ratio fluxes recorded
in the narrow and wide filters. This quantity shows an amplitude of 0.03 at
maximum. Due to the random fluctuations, no significant conclusions could
be deduced from the Hbeta index.




                        V. REGLERO

                        Departamento de Astronomia
                        Universidad de Valencia
                        Spain

                        J.J. FUENSALIDA, M.J. AREVALO

                        Instituto de Astrofisica de Canarias
                        Universidad de La Laguna

                        J.L. BALLESTER

                        Departamento de Fisica de la
                        Tierra y Cosmos

                        Universidad de Palma de Mallorca





References:

Andrews, A.D., Chugainov, P.F., Gershberg, R.E., Oskanian, V.S., 1969,
         I.B.V.S. 326
Gurzadyan, G.A., 1980, "Flare Stars" Pergamon Press [BIBCODE 1980flst.book.....G ]
Rodono, M., 1983, I.B.V.S. 2322
Rodono, M., et al., 1984, 4th European IUE Conference. Rome [BIBCODE 1984ESASP.218..247R ]