COMMISSION 27 AND 42 OF THE IAU
                   INFORMATION BULLETIN ON VARIABLE STARS
                               Number 3846

                                                         Konkoly Observatory
                                                         Budapest
                                                         29 February 1993

                                                         HU ISSN 0374 - 0676


               G172 - THE FOURTH HORIZONTAL BRANCH VARIABLE STAR
                      BLUEWARD OF THE RR LYRAE GAP IN M4

G172 (Greenstein 1939)=A522 (Alcaino 1975)=L1717 (Lee 1977) is a horizontal
branch star blueward of the RR Lyrae instability strip in the globular cluster
M4 (see Figure 1). The proper motion study (Cudworth 1990) shows G172 is a
cluster member (Pc=0.99). The magnitudes and colors given by different
authors are:

   V=13.17    B-V=0.35 (Alcaino 1975)
     13.14        0.56 (Lee 1977)
     13.29        0.52 (Cudworth 1990)

   The projection distance of the star from the center of the cluster is 38".
In Table 3 of his paper, Lee (1977) gave the note "V" for L1717 and pointed
out that it is a new RR Lyrae variable star not listed in Sawyer-Hogg's
catalogue. Lee did not determine the period of the star.
   We found the variability of G172 independently in 1976. We thought that
it was an RRc variable star at first. After measuring several hundreds of
photographic plates, we realized that it is not an RRc star because the
amplitude of the star never exceeds 0.1 mag. in B and the period is shorter
than 0.3 day if exists. Due to the low accuracy of photographic photometry we
could not determine the period of the star.
   Thanks to the use of the CCD camera, preliminary periods have now been
obtained. The observations were made with the RCA CCD #1 attached to the
Cassegrain focus of the 1-m Zeiss reflector at the Yunnan Observatory on 1992
May 6. The detector contains 320x512 pixels (pixel size 30x30micro m) at a scale of
about 0.47"/ pixel, thus covering a ~2.5'x4' field. A series of 270 second
exposures taken in rapid succession over a time interval of about 5.7h were
obtained (59 yellow, 2 blue). The seeing was between 2".0 and 3".3 (FWHM).
The data were reduced at the Zo-Se section of the United Laboratory for
Optical Astronomy with the Sun 4/65 workstation. The DAOPHOT (Stetson 1987) in
IRAF was used to reduce the data. The red star G248 (V=12.18, B-V=1.38) was
used as the comparison star. Comparing with the other stars in the frame, it
is shown that the constancy of G248 is better than ±0.006 mag. The
differential extinction between G248 and G172 has been corrected approximately.
Breger's (1991) program PERIOD was used to determine the periods. The results
so obtained are:

                           2
             m(t) = m_0 + Sum  a_isin(2*Pi*t/P_i + 2*Pi*Phi_i)
                          i=1

          Here P_1=0.0753d,  a_1=0.00894, Phi_1=0.7720,
               P_2=0.2117d,  a_2=0.00844, Phi_2=0.2059,
          m_0=1.142


                           [FIGURE 1]
                           [FIGURE 2]
                           [FIGURE 3]
                           [FIGURE 4]


   The "real time" light curve is shown in Figure 2 where the curve is
calculated with the formula above. The folded light curves are given in
Figures 3 and 4. Each light curve is plotted with the data prewhitened with
the other frequency.
   G172 is the fourth horizontal branch variable star in M4 which is located
blueward of the RR Lyrae gap and checked with the CCD photometry by us.
   Obviously, the periods given here are only preliminary, further observations
are needed to determine the periods accurately.
   I thank Chen Fu-xiang for help with observing. This work was partially
supported by the grant from the Chinese National Science Foundation.



                                                YAO BAO-AN

                                                ULOA/CAS
                                                Shanghai Observatory
                                                China



References:

Alcaino, G., 1975, Astron. & Astrophys. Supp., 21, 1. [BIBCODE 1975A&AS...21....5A ]
Breger, M., 1991, Delta Scuti Star Newsletter, issue 3.
Cudworth, K. M., 1990, A.J., 99, 1491. [BIBCODE 1990AJ.....99.1491C ]
Greenstein, J.L., 1939, Ap.J., 90, 387. [BIBCODE 1939ApJ....90..387G ]
Lee, S.W., 1977, Astron. & Astrophys., 27, 367. [BIBCODE 1977A&AS...27..367L ]
Stetson, P.B., 1987, P.A.S.P., 99, 191. [BIBCODE 1987PASP...99..191S ]