COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                   Number 1827

                                  Konkoly Observatory
                                  Budapest
                                  1980 August 4

                                  HU ISSN 0374-0676



         REVISED PHOTOMETRIC ELEMENTS OF XZ Sgr



    The eclipsing variable XZ Sagittarii is an Algol-type system
with a period of 3.28 days, composed of a main sequence A3 star
and a G5 subgiant. Photoelectric observations of this binary have
been performed by Knipe (1974) in V and B, and later by Kappelmann
and Walter (1979) in V, B and U. Knipe found some problems in the
solution of the light curves, mainly because of the unexpected absence 
of a normal reflection effect and the non-evidence of the
orbital eccentricity found spectroscopically by Sahade (1945,1949).
Suspecting that the problems that Knipe found were caused by gas
streams in the system, Kappelmann and Walter did not use any conventional 
method, but applied the procedure described by Walter
(1976) to take into account the absorption of the gas stream.
    The orbital elements obtained by both Knipe and Kappelmann
and Walter are however based on the rectifiable models of Kitamura
(1965) and Russell and Merrill (1952), respectively, which we know
have often led to misleading results (Mardirossian et al., 1980).
    Therefore we decided to obtain new geometric and photometric
elements for XZ Sgr, using the direct method developed by Wilson
and Devinney (1971), whose original computer code has been modified 
in order to run on a DEC PDP-11/34A minicomputer (Maceroni
et al., 1980). In this model, based on the Roche geometry, the
light curve is computed as a function of the following parameters:
i (inclination), Omega_{h,c} (surface potentials), T_{h,c} (polar temperatures), 
q=m_c/m_h (mass ratio), L_{h,c} (unnormalized monochromatic
luminosities), g_{h,c} (gravity darkening parameters), x_{h,c} (limb
darkening parameters), A_{h,c} (reflection parameters). The subscripts 
h and c refer to hotter and cooler component, respectively. 
Using the 75 normal points in V light given by Kappelmann and
Walter (1979) and performing the solution using the second of


                      [FIGURE 1]

the seven modes offered by the program (cfr. Leung and Wilson, 1977)
the following elements have been derived:
    i      = 88.5 +- 0.2     L/(L_h+L_c) = 0.1787
    Omega_h= 7.706+- 0.055   L_c/(L_h+L_c) = 0.8213+-0.0050
    Omega_c= 2.112+- 0.021   T_h = 9200^oK
    q      = 0.14 +- 0.02    T_c = 5150 +- 20^oK
    r_h(pole)= 0.1321        x_h = 0.53
    r_h(point)= 0.1324       x_c = 0.73
    r_h(side) = 0.1323       A_h = 1.0
    r_h(back) = 0.1324       A_c = 0.5
    r_c(pole)= 0.1994        g_h = 1.0
    r_c(point) = 0.2459      g_c = 0.32
    r_c(side)  = 0.2061
    r_c(back) = 0.2295

    The agreement with the solutions of the previous investigations 
is very good, but the absence of the rectification procedure 
ensures that effects such as reflection and polar brightening 
have been taken into the proper account. However, the not
very good fitting in the maxima that can be seen from the Figure,
validates the hypotheses of gas streams in the system and of a
hot region on the A star advanced by Kappelmann and Walter (1979).
New photoelectric and spectroscopic observations could allow a
more detailed study of these effects, not rare in Algol systems.




                          G. RUSSO and C. SOLLAZZO
                          Capodimonte Astronomical
                          Observatory,via Moiariello,16
                          I-80131 Napoli, Italy


References:
Leung, K.C. and Wilson, R.E.: 1977, Ap.J. 211, 853 [BIBCODE 1977ApJ...211..853L ]
Kappelmann, N. and Walter, K.:1979, Astron.Astrophys.Suppl.38,161 [BIBCODE 1979A&AS...38..161K ]
Kitamura, M.: 1965, Adv.Astron.Astrophys. 5, 27
Knipe, G.:1974, Monthly Not.Roy.Astron.Soc. 167, 369 [BIBCODE 1974MNRAS.167..369K ]
Maceroni, C., Milano, L., Russo,G. and Sollazzo,C.: 1980.Astron.
       J. (submitted)
Mardirossian, F.,Mezzetti, M., Cester,B., Giuricin C. and Russo,G.:
       1980, Astron.Astrophys.Suppl. 39, 235 [BIBCODE 1980A&AS...39..235M ]
Russell, H.N. and Merrill, J.E.: 1952, Contr.Princeton No.26 [BIBCODE 1952deeb.book.....R ]
Sahade, J.: 1945, Ap. J. 102, 470 [BIBCODE 1945ApJ...102..470S ]
Sahade, J.: 1949, Ap. J. 109, 439 [BIBCODE 1949ApJ...109..439S ]
Walter, K.: 1976, Astron.Astrophys.Suppl. 26, 227 [BIBCODE 1976A&AS...26..227W ]
Wilson, R.E. and Devinney, E.J.: 1971, Ap.J. 166, 605 [BIBCODE 1971ApJ...166..605W ]