COMMISSION 27 OF THE I. A. U.
       INFORMATION BULLETIN ON VARIABLE STARS
                    NUMBER 520

                              Konkoly Observatory
                              Budapest
                              1971 February 22

    ON THE CAUSE OF PERIODICAL LIGHT VARIATIONS
             OF SOME RED DWARF STARS

      At least three red dwarf stars are at present known
which show periodical light variations. Although all three
are spectrum binaries the light variations are definitely
not due to eclipses. The name of these stars, V magnitude
at minimum, amplitude and period of light variation,
absolute magnitude M_V and spectral class are given in the
Table.

----------------------------------------------
Name     V_min DeltaV     Period   M_V     Sp
----------------------------------------------
BY Dra    8.4   0.2       3.84d    7,6    K6Ve
CC Eri    8.7   0.3       1.56     8.4    K7Ve
FF And   10.4   0.06      2.17     8.7    dM0e
----------------------------------------------

      The photometric behaviour of these stars has been
studied by Evans, Chugainov and Krzeminski (1-4). It has
been found that the essential common features of them are
irregular variations of the amplitude and phase of the light
curve. Another important feature is the small amplitude of
colour variations. The change of the (B-V) colour during
one observational season usually does not exceed the errors
of observation. Changes of the (U-B) colour are not negligible
but these, probably, reflect the presence of some small
flare activity not unusual for red dwarfs.
      The existence of irregular variations of phase and
amplitude is the main argument contra the eclipse hypotheses. 
Beside these, as was found by Evans and Kraft, the
duration of the minimum in the case of eclipse would be
much shorter than observed because the separation of the
components is at least 10 times their radii.
      Krzeminski (4) has shown that the interpretation of
light variations by radial pulsations is not acceptable
because the periods are two order of magnitude greater than
those which were obtained on the base of the theory of red
dwarfs.
      The interpretation suggested by us (3) and developed 
by Krzeminski (4) is that the observed light variations
are caused by the presence of a spot on the surface of a 
rotating star. In order to obtain the observed amplitude of
light variation it is necessary to suppose that the area of
such a spot is about 10 percents of the stellar surface and


its effective temperature differs by several hundreds 
degrees from the temperature of the rest of the surface.
      Adopting this interpretation, one wants to know
whether the spot is bright or dark, The comparison of light
and colour variations in the case of BY Dra is interesting
in this respect. Plotted on Figure I for six observational
seasons are the upper and the lower limits of the B magnitude 
connected by vertical lines and the mean values of
the (B-V) colour (dots). We have obtained these data from
photoelectric observations obtained by us and other 
observers (3-5) in the period of 1954-1970. One finds from
these graphs that the (B-V) measures range from + 1.20
to + 1.24: In 1965-1968 the star was the reddest, the
amplitude of light variations the largest and the mean light
reached the minimum. In order to explain such a behaviour

                       [FIGURE 1]

of the star it is reasonable to suppose that the 
nonuniformity of the surface brightness was the largest in
1965-66 and it Was due to a decrease of the effective
temperature of the disturbed area.



                          P.F. CHUGAINOV

                     Crimean Astrophysical Observatory
                      U.S.S.R. Academy of Sciences


[References]
(1) D.S. Evans, Monthly Notices of the RAS, 119, 526, 1959. [BIBCODE 1959MNRAS.119..526E ]
(2) D.S. Evans, Monthly Notes of the ASSA, 23, 68, 1964. [BIBCODE 1964MNSSA..23...68E ]
(3) P.F. Chugainov, I.B.V.S., No. 122, 1966.
(4) W. Krzeminski, In "Low-Luminosity Stars", p. 57, Gordon
   and Breach, 1969, [BIBCODE 1969lls..symp...57K ]
(5) A. Masani, P. Broglia, E. Pestarino, Contr. dell'Osserv.
   Astron. di Milano-Merate Nuov, Ser., No. 59, 1955.
                      

      Editor's note:

       The continuous observation of Chugainov's stars
would be extremely important because diagrams like that
on the opposite page may reveal the existence of cycles
similar to the solar cycle in these stars.



                               L. DETRE