COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                  Number 2141

                                Konkoly Observatory
                                Budapest
                                1982 May 10

                                HU ISSN 0374-0676



       CONFIRMATION OF FLARE ACTIVITY ON G9-8
           BY PHOTOELECTRIC PHOTOMETRY



The nearby common proper motion stars G9-8 and G40-26 (Giclas
et al. 1971) were discovered photographically to be M dwarf
flare stars by Haro et al. (1975). They were subsequently
named CU Cnc and CV Cnc. At least the brighter star CU Cnc
shows the Halpha line in emission. The magnitude difference
between these two stars is DeltaV=1.39, and the fainter component
of the pair is slightly redder than the brighter one. At a
separation of about 12 arc sec it was possible for us to
isolate the brighter star for photoelectric high speed
photometry, using a 14 arc sec diaphragm.


                  TABLE I


       Photoelectric Flare Monitoring of G9-8


                               End(UT)   Obs. time
Date(UT)           Start(UT)             (sec)

14 November 1981   11:04:31   11:12:46    495
14 November 1981   11:14:52   11:43:40   1728
14 November 1981   11:44:52   12:04:31   1179
14 November 1981   12:05:52   12:19:22    810


16 November 1981   09:47:13   11:13:01   5148
16 November 1981   11:14:22   11:52:28   2286


    Total effective monitoring time = 11646 seconds

                                    = 3.235 hours

We monitored CU Cnc = G9-8 on 14 and 16 November 1981 UT with
the 2.1 m Struve reflector at McDonald Observatory. The
detailed observing log is given in Table I. A high speed
photometer controlled by a NOVA minicomputer (Nather 1973)
was attached to the telescope, and one second integrations
were taken succesively through eight filters. In each filter
the time resolution was nine seconds, due to the fixed time
needed for filter changes by the stepping motor. The 
bandpasses for which data are presented here were determined by
the response function of the RCA C31034A photomultiplier and
the transmission properties of the glass filters selected to
approximate the UBVR system. Typical measurement accuracies
at the one standard deviation level for the time series in
each of the filters were 0.07 (U), 0.014 (B), 0.008 (V), and
0.004 (R).

                       [FIGURE 1]

In 3.235 hours of effective monitoring time we detected one
flare. It was distinctly recorded only in the U-filter at a
peak amplitude of six standard deviations, but was marginally
detected also in the B-filter. No flare was seen in V or R at
an amplitude exceeding two standard deviations of the measurement
noise (Figure 1). The flare had a quick rise towards
maximum and decayed back to the preflare level over 15
minutes. This observation confirms the conclusion by
Haro et al. (1975) that CU Cnc is a flare star, and
represents the first photoelectric light curve of a flare
on this star.


It is a pleasure to thank the Director and Staff of
McDonald Observatory for support and hospitality during
my visit.




                      B. R. PETTERSEN
                      Institute of Mathematical
                      and Physical Sciences,
                      University of Tromso,
                      N-9001 Tromso, Norway.





References:
Giclas, H. L., Burnham, R., Thomas, N. G., 1971, Lowell
          Observatory Proper Motion Survey - The
          G Numbered Stars. [BIBCODE 1971lpms.book.....G ]
Haro, G., Chavira, E., Gonzalez, G., 1975, IBVS 1031.
Nather, R. E., 1973, Vistas in Astr. 15, 91. [BIBCODE 1973VA.....15...91N ]