COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 1624

                                   Konkoly Observatory
                                   Budapest
                                   1979 June 18




          SEMI-REGULAR VARIABLE STAR VII IN M13




     The variable star VII (Sawyer-Hogg, Publ.DDO, 3, No. 6,
1973) is one of the brightest and reddest stars in the globular
cluster M13=NGC 6205. It is undoubtedly physical member of the
cluster according to radial velocity measurements (Joy, ApJ.
110, 105, 1949), proper motions (Kadla, Iz.Pulkovo Obs., 24, 92,
1966) and position in the colour-magnitude diagram (Russev, Astr.
Zr., 51, 122, 1974).
     We have investigated the variability of the star VII 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 (J.D. 2442294-2443669). The photometric system was
close to the B one as it was reported recently (Russev and Russeva,
I.B.V.S. No. 1534, 1979). In addition to the present observational
material we have used the B measurements of Demers (AJ, 76, 445,
1971), Russev (VS, 19, No.2, 181, 1973) and Pike and Meston (MN,
180, 613, 1977).
     The analysis of the available observational data, covering
about 16 years (J.D. 2437790-2443669), allowed us to establish,
(1) that the period P=91.77d, obtained by Russev (VS, 19, No. 2, 181,
1973), presents only the fundamental period of VII and (2) that
the light curve is probably subjected to periodical changes, 
manifested by changing the roles of the main and secondary maxima with
a period pi=2070d. As it is shown in the Figure, where the size of
the symbols depends on the number of the observations per night,
during 1035d the star has a light curve (Figure a), which is
characterized with a secondary maximum (bump) on the descending
branch at phase Phi =0.45. During the following 1035d the secondary
maximum is transformed in a main one, and the former main maximum

                             [FIGURE 1]

decreases and becomes a bump of the descending branch of the light
curve (Figure b) and so on. The data allowed us to study six such
changes, the moments of which are obtained by the formula:
        T = J.D. 2436370 + 1035d*N.
The odd N gives the beginning of the intervals in which "acts"
the first maximum with the following elements of the light curve:
        Max = J.D. 2438228.85 + 91.77d*E
When N is even we have respectively the elements.
        Max = J.D. 2438272.44 + 91.77d*E 
It seems that the change of the maxima takes place comparatively
quickly, for about 1-2 fundamental periods. The next such change
of the VII light curve may be expected at the beginning of 1981.
     We have obtained from the light curve for VII B=13.45m and
amplitude of the variation A_B=0.68m. Since V=11.83m (from the 
observations of Demers, Pike and Meston and two our V plates)
the colour index of the star is B-V=+1.62m.
     The red semi-regular variables with periods about 90 days
in globular star clusters are not a rarity. We may show at least
6 stars with periods from 90 to 93 days (V2 NGC 362, V148 NGC 5139,
V17 NGC 6626, V5 NGC 6656, V6 NGC 6779, V19 NGC 7006), which are
similar to VII in M13. Doubtlessly they have something in common
with the RV Tau type stars of the Galaxy field, but their connection 
with them, at present, is not sufficiently investigated.
     The details of our studies of VII together with other
variables in M13 will be published elsewhere.




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