COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                       NUMBER 597

                                    Konkoly Observatory
                                    Budapest
                                    1971 December 2




             FLARE MONITORING OF AD Leo


    One flare was recorded during 7.61 hours of ultraviolet monitoring 
within the scheduled April 1971 period for cooperative X-ray and
optical observations (IBVS 516). A night in March was spent tracing
possible instrumental causes for the fact that three of the six AD Leo
"flares" reported in IBVS 499 occurred at the same telescope position.
No instrumental causes were confirmed, and, in fact, the reality of one
of the flares in question has been supported by the independent observations
of Cristaldi and Rodono, IBVS 498. They were not observing at the times
of the other flares.
    Table I shows the actual monitoring times rounded to the nearest
minute of UT. In all cases, an EMI 62565 photomultiplier with d.c. 
amplification (Keithley 416) and direct strip-chart recording (Speedomax W) was
used with the 61-cm diameter Cassegrain at Mt. Cuba Observatory. On April
15 and 16 a standard U filter (Corning 7-54) was used with the f/16
secondary mirror. On March 25, in order to duplicate the 1970 observing
conditions, the calcium K-line filter was used with the f/32 secondary.
    Although all lighting and physical settings on March 25 were made to
agree as closely as possible with the 1970 conditions, there was no sign
of a brightened signal occurring between hour angles - 1:56 and - 1:36 (or
at any other time during that night). Of the suspected flares reported
in IBVS 499, flare No. 3 was the most dubious. This, however, is the only
one for which I know of simultaneous monitoring having been done elsewhere.
It has the same time of maximum (1970 March 8, 2h 27.3m UT) as the second
of the four flares reported by Cristaldi and Rodono in IBVS 498. They also
noted it to be uncertain and gave the blue-light brightness and duration
as several times smaller than my estimate at the K line. This is a reasonable
degree of dissimilarity for the difference between the filters used. That
there remain two unconfirmed flares at nearly the same hour angle is no
longer a basis for regarding them as probably spurious.




       Table I. Coverage of AD Leo during 1971.
-------------------------------------------------------
Date                    UT Coverage
-------------------------------------------------------
March 25   1h09m - 13m,1:17-37,1:43-58,2:01-19,2:23-43,
           2:47-3:03,3:06-36,4:23-57,5:02-16.
April 15   2:30-42,2:44-53,2:57-3:18,3:19-23,3:25-35,
           3:37-43,3:46-4:03,4:07-21,4:23-40,4:42-59,
           5:03-25,5:29-47,5:49-6:21.
April 16   2:12-31,2:43-3:06,3:10-11,3:16-29,3:38-54,
           3:56-4:12.
-------------------------------------------------------




    Table II. Photometry of AD Leo during quiescence.
-------------------------------------------------------
1971                           I_0/sigma
Date      Filter   UT  m_c - m_0       m_lim - m_0
-------------------------------------------------------
March 25   Ca-K   1:11   1.31      6.8    1.77
                  2:33   1.32      6.7    1.75
                  4:46   1.26      6.2    1.66
April 15   U      3:08   1.09      8.6    2.01
                  3:40   0.99      8.7    2.03
                  5:18   1.10      6.8    1.76
April 16   U      2:51   1.25      5.9    1.61
                  4:01   1.23      5.5    1.52
--------------------------------------------------------


                         [FIGURE 1]

     Quantities descriptive of the single flare observed this year
are given in the figure, along with an unsmoothed reproduction of the
raw data from the photometer chart. All of these quantities are as
defined in IBVS 499, although the optical filter is different. Signal
to noise is estimated both for the instrumental time constant (I_0/sigma)
and for a five-times-longer time constant (appropriate to the mental
smoothing that is done in looking for flares) to approximate the minimum
detectable flare magnitude (m_lim - m_0).
     Table II provides a sample of the variability (with respect to
the comparision star identified in IBVS 449) of AD Leo at times devoid
of identified flares.




Mt. Cuba Observatory        RICHARD B. HERR
1971 November 23            University of Delaware
                            Newark, Delaware
                            U. S. A.



                   REFERENCES

A.D. Andrews, P.F. Chugainov, V.S. Oskanian, and E.M. Kellogg,
      IBVS No. 516, 1971.
S. Cristaldi and M. Rodono, IBVS No. 498, 1970.
R.B. Herr, IBVS No. 499, 1970.