COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                       Number 2519

                                         Konkoly Observatory
                                         Budapest
                                         10 May 1984

                                         HU ISSN 0374-0676


       SPECTROPHOTOMETRY OF PECULIAR SHELL STAR HD 218393



  HD 218393 is a well known shell star having several interesting spectral
features. Struve (1944), Merrill (1949), Holliday (1950) and Doazan and
Peton (1970) observed this star spectroscopically and noticed a series of
changes in the spectrum. The radial velocity variations of the circumstellar
lines suggested a period of 35 to 40 days but the amplitude was not exactly
the same. Doazan and Peton (1970) interpreted the radial velocity variations
as caused by an expanding envelope with variable accelerations, while after
analysing all the available radial velocity data of HD 218393, Kriz and
Harmanec (1975) suggested that the object may be an interacting binary with
a possible orbital period of 38.873d. Later on, Polidan and Peters (1976)
 proved the binary nature of the star by the discovery of some lines of the
secondary component in the infrared part of the spectrum. According to them,
the system consists of a B3e primary and gK1 secondary. Harmanec et al. (1977)
observed this star photometrically and discovered its photometric variability
of 0.3m, 0.1m and 0.1m, respectively, in U, B and V filters. They observed a
gradual decline taking place during about 10 days in 1974 and more rapid
variation during 1976.
  We observed HD 218393 spectrophotometrically in the wavelength range
lambda lambda 3200-8000 A with the Hilger and Watts scanner during two nights
on 12 and 13 November 1983. The scanner was mounted at the Cassegrain focus of
the 104-cm reflector of Uttar Pradesh State Observatory. In addition, the stars
alpha Lyr and xi^2 Cet were observed as the standard stars while the stars 2
And (A3V) and 8 Cyg (B3IV) were observed as the comparison stars. All the scans
were obtained with 50 A band pass. The observational techniques and the
data reduction procedure were the same as used earlier (Goraya, 1981). The
absolute monochromatic fluxes of Be star and comparison stars were extracted
from the observed continuous energy distributions at about 49 wavelengths
separated by 100 A in, the whole observed spectrum. The monochromatic fluxes
were corrected for interstellar reddening and were normalised to lambda  5500 A.
A plot of the mean monochromatic fluxes against the wavelength is shown in
Figure 1.


                               [FIGURE 1]

 Figure 1   Observed continuous energy distribution curve of HD 218393 
 compared with the early type star 8 Cyg (B3IV) and the late-type star
 2 And (A3V).


  We compared the observed energy distributions of 2 And and 8 Cyg with
Kurucz's (1979) model atmospheres. We found that the observed curves of 2 And
and 8 Cyg fit well with models having Teff =8300K and 20000K, respectively
over the whole spectral range. Surprisingly we noticed that the observed
energy distribution curve of HD 218393 could not be fitted with any model.
It is clear from Figure 1 that the observed curve of HD 218393 matches with
8 Cyg in the near ultraviolet and Balmer discontinuity region (lambda lambda
3200-4200 A).
Some part of Paschen continuum region (lambda lambda 4200-6000 A) of HD 218393
matches well with 2 And. A near infrared excess longward of lambda 6000 A was
also noticed for HD 218393. From this type of peculiar continuum energy
distribution we infer that the continuous spectrum of HD 218393 is a
combination of the spectrum of hotter primary component (early B-type) and
the spectrum of secondary cooler companion (K-type).
  The observed anomalies in the continuous energy distribution of HD 218393
lead to suggest that the object is an interacting binary star, consisting of
B type primary and K type secondary, in which the hotter K star is heavily
obscured by a gaseous disk or ring. Outside the star, the disk itself
radiates, usually as an optically thin hydrogen cloud. This continuous radiation
of the disk becomes relatively stronger as we proceed towards longer
wavelengths in the optical region, resulting in a near infrared excess
emission.


         P.S. GORAYA, MAHENDRA SINGH, U.S. CHAUBEY

            Uttar Pradesh State Observatory,
            Naini Tal-263129, India

References:

Doazan, V. and Peton, A.: 1970, Astron.Astrophys. 9, 245 [BIBCODE 1970A&A.....9..245D ]
Goraya, P.S.: 1981, Astrophys, Space Sci. 78, 419 [BIBCODE 1981Ap&SS..78..419G ]
Harmanec, P., Horn, J., Koubsky, P., Kriz, S., Ivanovic, Z., and
      Pavlovski, K.: 1977, IAU Inf.Bull. Variable Stars No. 1324
Holliday, J.: 1950, J.Roy.Astron.Soc. Canada, 44, 149 [BIBCODE 1950JRASC..44..149H ]
Kriz, S. and Harmanec, P.: 1975, Bull.Astr.Inst.Czech. 26, 65 [BIBCODE 1975BAICz..26...65K ]
Kurucz, R.L.: 1979, Astrophys.J.Suppl. 40, 1 [BIBCODE 1979ApJS...40....1K ]
Merrill, P.W.: 1949, Astrophys.J. 110, 420 [BIBCODE 1949ApJ...110..420M ]
Polidan, R.S. and Peters, G.J.: 1976, IAU Symp. No. 70, 59 
      ed. A. Slettebak, D. Reidel Publ. Co. Dordrecht [BIBCODE 1976IAUS...70...59P ]
Struve, O.: 1944, Astrophys. J. 99, 75 [BIBCODE 1944ApJ....99...75S ]