COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1126

                                Konkoly Observatory
                                Budapest
                                1976 April 15



              EVIDENCE FOR A BUMP IN THE LIGHT CURVE OF
                           VARIABLE 1 OF M13


  That there is a relationship between the pulsation period of population
I cepheid variables and the occurence of a secondary bump in the light curve
has long been known (Hertzsprung 1926, B.A.N. 3, 115). Recent theoretical
calculations predict that a similar relationship should exist for the 
population II cepheids with periods in the range 1-3 days and Stobie (1973, 
Observatory 93, 111) showed that bumps do occur in the light curves of seven halo
and old disk cepheids with periods in this range, including two globular
clusters members.

    The importance of the detection and observation of the bumps is that a
determination of the phase at which the bump occurs permits the mass and
radius of the star to be calculated. Cepheids with bumps in globular clusters 
are particularly important since the derived masses and radii can be
compared with similar values for cepheids or horizontal branch stars determined 
by independent methods.

  Variable 1 of the globular cluster M13 has a period in the range where
a bump is predicted and indeed there is some evidence for a bump in the m_pv
light curve of this star published by Arp (1955, A.J. 60, 1). In an attempt
to confirm if the suspected bump really does exist we have collected together
all the published photometric observations of M13-Var.1 and plotted the
light curves of the various authors using the same period. The data are not
very satisfactory, most being eye estimates of the photographic magnitudes.
Nevertheless, the observations of Sawyer (1942, Publ. David Dunlap Obs. 1,
231), Kollnig-Schattschneider (1942, A.N. 273, 145) and Osborn (1969, A.J.
74, 108) show evidence for a bump in the light curve near phase 0.8. The
observations of Shapley (1915, Mt. Wilson Contrib. 6, 301), Demers (1971,
A.J. 76, 445) and Russev (1973, Variable Stars 19, 181) either have too few
observations or do not cover the appropriate part of the light curve and give
no evidence one way or the other. However, the fact that all the apparent
bumps occur at approximately the same phase as the bump seen in Arp's more
accurate light curve from iris photometry indicates that the feature is 
probably real.

  Assuming that there is a bump in the light curve of M13-Var.1 at phase
0.8 we can use the equations given by Stobie to calculate the stellar radius
and mass. The results are R= 5.8 R_sun and M= 0.2 M_sun. The values are somewhat
smaller that those found for the other population II cepheids studied by
Stobie but they agree with the mass and radius of this star calculated by
Bohm-Vitense et al (1974, Ap. J. 194, 125), who also commented on the 
apparently anomalously low values. It is interesting that Osborn's masses for
two asymptotic branch stars in M13 were also very small. It is obvious that
the determination of a good photoelectric light curve for this star would be
worthwhile.




           WAYNE OSBORN        PATRICIA ROSENZWEIG
           Fundacion CIDA      Facultad de Ciencias
           Merida, Venezuela   Universidad de Los Andes
                               Merida, Venezuela