COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1534

                                Konkoly Observatory
                                Budapest
                                1979 January 18




           RED VARIABLE STAR L973 IN M13


    Recently it was shown by Fuenmayer and Osborn (IBVS 952,
1974) that one of the brightest and reddest stars in M13 =
NGC 6205, no. 973 in Ludendorff's catalogue (Publ. Potsdam
Obs., 15, no. 50. 1905), which as it was suggested by Russev
(Astr. Zr., 51, 122, 1974) to vary in brightness, is indeed a
variable star. It is a member of M13 according to radial velocity 
measurements (Noris and Zinn, 1977, ApJ, 215, 74).
    We have investigated the variability of the star L 973 on
43 blue plates of M13 taken with the 60-cm reflector of the
Belogradchik Astronomical Station (Bulgaria) during four years,
from 1974 to 1978 (JD 24 42 294 - 43 724). The plates were
ORWO - ZU2 emulsion in combination with ultraviolet filter
(Panchromar UVII) which give a system close to the B one. Using
the photometry of Cathey (AJ, 79, 1370, 1974) we have measured
the magnitudes of the star with a MF - 2 photometer. The average
error of the measures was +-0.07m. In addition to the present
observational material we have utilized 19 blue plates from the
collection of the State Astr. Inst. Sternberg in Moscow 
obtained with the 70-cm reflector AZT-2, during 1961 - 1972 (JD
24 37 790-41 092). The accuracy of the Moscow magnitudes, which
have been measured with an automatic iris-photometer, is close
to our measurements. Moreover we have used the observations of
Fuenmayer and Osborn (IBVS 952, 1974).
    The available observational data covering about sixteen
years have permitted us to obtain confidently seven season
maxima of the brightness, which indicated that the period of
variability was approximately 43.3 days and that this one 
itself is probably not constant. Applying the usual method we
  have derived the following elements of the light curve:

      Max = JD 24 38 246.4 + 43.49d.E (to JD 24 40 716.6)
      Max = JD 24 41 069.6 + 43.04d.E (after JD 24 40 716.6)

      The light curve of L 973 in M13 constructed by the upper
  elements is shown in the Figure. The symbols are indicated in
  the Figure. Their size depends on the number of observations
  per night.
      Besides we have attempted to explain the season maxima
  with a continuously changeable period and the result is:
  
    Max = JD 24 38 248.53 + 43.515d.E - 0.00225.E^2

      The light curve constructed with this ephemeris is quite
  similar to the one presented in the Figure, but the residuals
  O - C, in general, are larger compared to those obtained with
  the first elements.

                           [FIGURE 1]

       In general it follows from both interpretations that the
  period of the star during the last sixteen years has probably
  decreased with 0.45 days.
       From the average light curve we have obtained for L 973
   B = 13.53m and the average amplitude of the variation A_B = 0.45m.
   Since V = 12.04m (from 5 Moscow plates, this magnitude has been
also given by Osborn 1971, Ph.D. Thesis, Yale Univ.) the 
colour index of the star is B - V = +1.49m.
    Finally, it is important to point out that red variables
with periods about 43.3 days are met very rarely in the globular 
clusters. Our attempt to compare L 973 with similar objects
in other globular clusters shows that the number of this kind
of variables discovered up to now is not more than four
These are first of all V53 (P = 32.7d) and V164 (P = 37.2d) in
omega Cen, V13 (P~40d) in NGC 6121 and V6 (P = 47d) and V18 (P ~
49:d) in 47 Tuc.
     The details of our studies of L 973 together with other
red variables in M13 will be published elsewhere.



                    RUSCHO RUSSEV and TATIANA RUSSEVA
                        Department of Astronomy
                          University of Sofia
                            Sofia, Bulgaria