COMMISSIONS 27 AND 42 OF THE IAU
            INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3920

                                               Konkoly Observatory
                                               Budapest
                                               9 August 1993

                                               HU ISSN 0374 - 0676


            IS K1098 IN M15 A SHORT PERIOD RED VARIABLE STAR?



 The red star K1098 (Kustner, 1921) in the globular cluster M15 was found by Yao to
be a new suspected variable star. Now we have checked it with CCD photometry and
confirmed its variability. According to our determination, its V=15.67m and B-V=0.97m.
The coordinates of the star relative to the cluster center are: x=332", y=90". The location
of the star on the C-M diagram is shown in Fig. 1. Neither radial velocity nor proper
motion determinations are found in literature. Maybe it is a field star.
 The star was observed by us with the RCA CCD #1 attached to the Cassegrain focus
of the Zeiss 1-m reflector (f/13.3) at the Yunnan Observatory in December 1992 and with
the RCA CCD #2 at the same telescope in May 1993. The CCD #1 contains 320x512
pixels at a scale of 0.47"/pixel, thus covering a 2.5` x 4' field and the CCD #2 512 x 512
pixels at the same scale covering a 4'x4' field. A total of 31 yellow and 4 blue 600 second
exposures were obtained. The seeing was between 2.0" and 4.0" (FWHM). The reduction
process was identical to that used before (Yao, 1990, 1993). The results are:

            m(t)=m_0 + Summa_i=1^2 a_i sin(2pi*t/p_i + 2pi* phi_i)

 Here p_1=0.5155d,     a_1=0.0566, phi_1=0.0050
      p_2=0.0927d,     a_2=0.0193, phi_2=0.3727

 The m_0 represents the mean magnitude difference between K1098 and the 
comparison star K1073, which is also a red star of similar color.
 The folded light curves are given in Figures 2 and 3. Each light curve is plotted with
the data prewhitened with the other frequency. Here the circles represent the observations
on 16 December 1992, the open triangles on 17 December 1992, the x's on 23 May 1993,
the squares on 24 May 1993 and the filled triangles on 26 May 1993.
 The interstellar reddening in the direction of M15 is not large (E(B-V)=0.12), so the
star K1098 is intrinsically red. Why does a red star pulsate with a short period? How
long can the pulsations of these kinds persist?
 Further observations are needed to determine the periods accurately and to check their
possible variations.

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

 This work was partially supported by the grant from the Chinese National Science
Foundation.



                         YAO BAO-AN
                         ULOA/CAS
                         Shanghai Observatory
                         China

                         QIN DAO
                         Purple Mountain Observatory
                         China


References:
Kustner, F., 1921, Veroff. Sternw. Bonn, No. 15
Yao Bao-an, 1990, IBVS, No. 3431
Yao Bao-an, 1993, IBVS, No. 3846