COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                       Number 3310

                                         Konkoly Observatory
                                         Budapest
                                         24 March 1989

                                         HU ISSN 0374 - 0676


    THE DEPENDENCE OF THE EFFECTIVE TEMPERATURE UPON THE RADIUS
         VARIATION IN THE PULSATING VARIABLE STAR XZ Cyg


The Blazhko effect in the pulsating RR Lyrae - type variable stars can be
interpreted by the interference of primary and perturbing variations in the
radius (Zessevich, 1970; Romanov, 1975). In order to study the nature of the
Blazhko effect it is of interest to consider the relationship between the main
stellar parameters, Te and R, responsible for the luminosity of the star and
varying with the pulsation process.

Based upon observations the dependence of Te on the radius variation of XZ Cyg
is considered for four phases psi covering a whole period of the Blazhko
effect. When constructing the dependence (T_e-Delta R)_psi the results of
simultaneous photoelectric (Kinchev, 1974) and spectral observations (Romanov
and Fenina, 1981) were used.

The effective temperature was determined from the B-V colour index corrected
for the interstellar absorption. The interstellar absorption in the direction
of XZ Cyg was taken equal to 0.05 mag (Romanov and Fenina, 1981).

The B-V colour index was converted into effective temperature by means of the
effective temperature scale determined by Zaikova and Romanov (1978). When
transforming B-V into T_e the ratio of the heavy element abundance of XZ Cyg
(Z) and the population I stars (Z_I) was considered to be equal to 0.7 (Gadun
et al., 1982). The influence of the shock wave effects was not taken into
account.

The curves of radius variation delta R in XZ Cyg at the level of continuous
spectrum formation for four intervals of the Blazhko effect phases were
constructed by transforming the radius curve variation Delta R_H at the level
of hydrogen absorption line formation. It was supposed that the ratio of the
amplitudes of the radius variation on that level was equal to 1.6 (Romanov,
1977), whereas the phase of the minimum radius coincided with that of the
origin of the ascending branch of the light curve (Romanov, 1974). The radius
variations were obtained by Kinchev and Romanov by integrating the curves of
radial velocity variations constructed from measurements of the hydrogen lines.


                                [FIGURE 1]

 Figure 1. The dependence of the effective temperature upon the radius 
 variation with the phase of the primary period. Phases of the primary 
 period are plotted with the interval of 0.05 P , the values of some of 
 these being indicated in the diagram for four phases of Blazhko effect: 
 psi=0.0 (rectangles), psi=0.9 (crosses), psi=0.6 (open circles), psi=0.5 
 (filled circles).


The (T_e-Delta R)_psi diagram is given in Figure 1. The zero point of the 
delta
R scale coincides with the mean stellar radius. The diagram shows that the
dependences T_e - Delta R represent closed loops. The loops near the Blazhko
effect maximum cover a large area in the diagram and fully cover the loops near
the Blazhko effect minimum.

The differences between the T_e values for the brightness minima at different
phases of the Blazhko effect are insignificant, and when Z/Z_I=0.7, the mean
value of T_e is about 6400 K, this does not overlap the T_e values of the red
edge of the instability strip for RR Lyrae - type stars (Deupree, 1977).

On the other hand, the decrease of the minimum radius, while the star is
compressed, is followed by an increase of the amplitude of the effective
temperature variation and the loop area in the diagram.

Future series of simultaneous photoelectric and spectral observations will make
it possible to specify the set of dependences (T_e-Delta R)_psi and use it for
comparing with model calculations of pulsating RR Lyrae - type variable stars.


D.E. MKRTICHIAN and Yu.S. ROMANOV
Odessa Astronomical Observatory
Odessa, 270014, Shevchenko Park
USSR

References:

Deupree, R.G. 1977, Astrophys. J., 214, 502. [BIBCODE 1977ApJ...214..502D ]
Gadun, A.S., Zaikova, L.P., and Romanov, Yu.S. 1982, Astrometry and
   Astrophysics (in Russian), 46, 23. [BIBCODE 1982AAfz...46...23G ]
Kinchev, P.Z. 1974, Variable Stars, 19, No. 5, 477. [BIBCODE 1974PZ.....19..477K ]
Romanov, Yu.S. 1974, Dissertation.
Romanov, Yu.S. 1975, in:"Multiple Periodic Variable Stars", IAU Coll.
   No. 29, Budapest, Vol. 2, 191. [BIBCODE 1976mpvs.conf..191R ]
Romanov, Yu.S. 1977, Variable Stars, 20, No. 4, 299. [BIBCODE 1977PZ.....20..299R ]
Romanov, Yu.S., and Fenina, Z.N. 1981 Variable Stars, Suppl., 4, No. 19, 35. [BIBCODE 1981PZP.....4...35R ]
Zaikova, L.P., and Romanov, Yu.S. 1978, Astrometry and Astrophysics, 35, 76. [BIBCODE 1978AAfz...35...76Z ]
Zessevich, V.P. 1970, in: "Pulsating Stars", B.V. Kukarkin ed., p. 222.