COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1333

                                  Konkoly Observatory
                                  Budapest
                                  1977 September 1




        REVISED PERIODS FOR TWO RR LYRAE STARS :
    VARIABLE 24 IN NGC 6171 AND VARIABLE 23 IN NGC 6656





    The globular clusters NGC 6171 (Messier 107, R.A. 16h29.7m,
Dec.-12d57',1950) and NGC 6656 (Messier 22, R.A. 18h33.3m, Dec.
-23d58', 1950) have been studied as part of the program at the
David Dunlap Observatory for more than 30 years. Now they have
been observed with two telescopes at the Las Campanas Observatory 
of the Carnegie Institution of Washington. Both of the stars
investigated here are in such crowded areas that it is not 
possible to determine magnitudes very accurately. This makes period 
determination difficult. The northern hemisphere observations
could be made over only a short interval in one night, and from
these the variables were thought to be of type c. However, the
present study, with nine hour runs on some nights, shows that
both are of type a. On Las Campanas, they have been photographed 
with the University of Toronto 24-inch telescope since 1972
and NGC 6656 has also been photographed with the Carnegie 40 inch
on five consecutive nights in 1976.
    Variable 24 in NGC 6171 was discovered by Oosterhoff (1938)
who published magnitudes from 15 photographs but did not have
sufficient data for period determination. A period of 0.3462153
days was determined by Coutts and Sawyer Hogg (1971) who commented 
that an alternate value 0.529586 days was possible. Our Las
Campanas plates have been measured with a Cuffey iris astrophotometer 
and reduced with the aid of sequence by Sandage and
Katem (1964). This study indicates that the period is approximately 
0.523 days and is changing. We therefore include only
1977 observations in the light curve. A more detailed study of
the period changes of this and other RR Lyrae variables in NGC
6171 will be presented in a future paper.

                      [FIGURE 1]

                      [FIGURE 2]

     Variable 23 in NGC 6656 was discovered by Sawyer (1944)
who also determined a period, 0.3557 days. She commented that
this variable was one of the most difficult in the cluster to
estimate owing to its position near the centre. In the present
study, we have estimated the magnitudes by eye, using a sequence
by Arp and Melbourne (1959). A period of 0.55076 days fits all
the observations presented here. This period is accurate to two
figures, but further observations will be necessary to determine
reliably the last three figures. On the light curve, open circles
represent the 40-inch observations and closed circles the 24-inch
observations.
     This research has been supported by a grant through the
National Research Council of Canada. We are grateful to Dr. Amelia
Wehlau for suggesting we investigate variable 23 in NGC 6656.



           CHRISTINE COUTTS CLEMENT *     HELEN SAWYER HOGG
           Department of Astronomy        David Dunlap Observatory
           University of Toronto          Richmond Hill, Ontario
           Toronto, Ontario, Canada       Canada
           M5S 1A7                        L4C 4Y6

* Guest observer Hale Observatories, 1976





References:
Arp, H.C. and Melbourne, W.G. 1959, A.J. 64, 28 [BIBCODE 1959AJ.....64...28A ]
Coutts, C.M. and Sawyer Hogg, H. 1971. Publ.David Dunlap Obs. 3, 61 [BIBCODE 1971PDDO....3...62C ]
Oosterhoff, P.Th. 1938. B.A.N. 8, 273 [BIBCODE 1938BAN.....8..273O ]
Sandage, A. and Katem, B. 1964. Ap.J. 139, 1088 [BIBCODE 1964ApJ...139.1088S ]
Sawyer, H.B. 1944. Publ.David Dunlap Obs. 1, 297 [BIBCODE 1944PDDO....1..294S ]