COMMISSIONS 27 AND 42 OF THE IAU 
               INFORMATION BULLETIN ON VARIABLE STARS 
                            Number 3757 
 
                                          Konkoly Observatory 
                                          Budapest 
                                          10 August 1992 
 
                                          HU ISSN 0374 - 0676 
 
 
            Photometric orbit of the triple system DI Pegasi 
 
  The Algol-type eclipsing binary DI Peg is a relatively well observed 
system.  Rucinski (1967) was the first who pointed out that the light 
curve solution of this close binary needs a relatively large amount of 
third light (24 % of total luminosity), therefore DI Peg belongs to a 
triple system.  Recently, Lu (1992) presented detailed spectroscopic and 
photometric analysis of the system and confirmed the existence of the 
third body from cross-correlation spectra.  He also noted that light time 
effect may be present in the O-C diagram. 
 
  The aim of the present study was to estimate the parameters of the 
absolute orbit of the close pair around the centre of mass of the triple 
system from the O-C diagram. only the times of minima obtained from 
photoelectric photometry were used (for the list of references, see Lu 
(1992)).  The ephemeris derived by Lu (1992) was adopted: 
 
    Min I HJD = 24 25918.3597 + 0.71181663d E           (1) 
 
 
 
  A Keplerian orbit was assumed for the wide system.  The orbital 
equations were fitted to the O-C diagram (see e. g. Mayer, 1990) with 
the method of least squares.  Note that the dynamical perturbation 
discussed by Mayer (1990) is negligible in this case because of the long 
third-body period.  The results of the orbital solution are summarized in 
Table I. The given parameters are the projected semi-major axis, the 
eccentricity, the longitude of the periastron, the time of periastron 
passage, the orbital period and the mass function, respectively. 
 
 
                                 Table I 
 
              Orbital elements of the triple system DI Peg 
 
a sin i (10^6 km)      115 +/- 31 
                e     0.66 +/- 0.2 
      omega (rad)      2.5 +/- 0.4 
         tau (JD)  2440612 +/- 400 
   P_orb (days)       8070 +/- 500 
 f(m2) (M_sun)       0.001 +/- 0.001 

  Fig. 1. shows the O-C diagram with the fitted curve (top
 panel) and the calculated orbital radial velocities of the close
 pair around the common centre (bottom panel).

                           [FIGURE 1]

    Figure 1   O-C diagram and the fitted orbital curve 
 

  Since the 3rd star is roughly as luminous as the components of the 
close pair, the mass of the visual companion must be close to 1 M_sun 
if it is a main-sequence star (the masses in the eclipsing pair are 1.2 
and 0.7 M_sun, (Lu, 1992)). This means that the inclination of the 
wide orbit should be 12deg +/- 2deg, which gives about 10 AU as the absolute 
value of the semi-major axis of the relative orbit of the close pair and 
the 3rd body. 
 
  The orbital parameters derived here are quite uncertain, because of the 
shortness of the observed time interval. Further observations may lead 
to more precise and complete analysis of this system. 
 
 
 
Acknowledgement 
 
  The author is very grateful to S.M. Rucinski for collecting and sending 
the O-C data. 
 
 
 
                             J. VINKO 
 
                     Dept. of Optics, JATE University 
                    SZEGED, Dom ter 9. H-6720, Hungary 
 
References 
 
Lu, W. 1992, Acta Astr. 42, 73.  [BIBCODE 1992AcA....42...73L ]
Mayer, P. 1990, Bull. Astron. Inst. Czechosl. 41, 231.  [BIBCODE 1990BAICz..41..231M ]
Rucinski, S.M. 1967, Acta Astr. 17, 271.  [BIBCODE 1967AcA....17..271R ]