COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 2800

                                         Konkoly Observatory
                                         Budapest
                                         1 October 1985

                                         HU ISSN 0374 - 0676




      U OPHIUCHI: AN ECLIPSING BINARY WITH RAPID APSIDAL MOTION

                  (BAV-Mitteilungen Nr. 40)



  U Ophiuchi is a detached system, with well-determined masses and absolute
dimensions. The period variations are an intriguing characteristic of the 
variable, and an explanation in terms of light-time effect in a possible triple
system was put forward already by Parenago (1949). A truly periodic representation,
however, still remains somewhat unconvincing (Herczeg, 1980). Difficulties 
are also manifest in diverging values of the parameters for the light
time orbit given by different authors (e.g., Koch and Koegler, 1977, 
Panchatsaram, 1981).
  A clue to these problems might be sought in the observed times of Min II,
that follow roughly the trend of the primary minima, but with systematic 
deviations. As already noticed by Koch and Koegler, shorter - term fluctuations
on a time scale of ~= 20 yr are apparent in the phase of secondary eclipse 
relative to mid-primary. Yet the authors ruled out an explanation based on 
apsidal motion. The reason was that the variations seemed to lack rigorous 
periodicity; besides, there seemed to be "a clear preference for secondary 
eclipse to occur before the half-period moment".
  A thorough period study of the available photoelectric minima times 
(including a reanalysis of the early photoelectric observations by Huffer 
(Huffer and Kopal 1951) and Magalashvili (1999)) has convinced us, however, that
their conclusions were based on scanty and incomplete data. In Fig. 1 we 
present interpolated values of the quantity (tII - tI - P/2) for the observed 
secondary minima. A cosine fit to these data gives an entirely satisfactory 
approximation. Straightforward interpretation in terms of a rotating eccentric
close orbit yields the preliminary values of

        e = 0.0032 +/- 0.0002,        U = 20.7 +/- 0.3 yr

for orbital eccentricity and period of apsidal revolution, respectively.
 After subtraction of the phase shifts due to apsidal motion, the O-C plot


                            [FIGURE 1]

 Figure 1: Time variation of the displacement of secondary minimum
           of U Oph relative to mid-primary. The continuous curve
           is the function 0.0034d cos (337d + 17.38d (t - 1969.72)).

                            [FIGURE 2]

 Figure 2: (O-C) diagram of the observed times of primary minimum
           1881 - 1985. Modified after Frieboes-Conde and Herczeg
           (1973), Fig. 19. The continuous curve is the predicted
           O-C curve for Min I according to our final elements for
           light-time orbit and apsidal motion.




was compared with a grid of standard light-time curves. Apsidal motion 
parameters and light-time orbit were adjusted by means of a differential 
corrections procedure. The final set of elements so obtained satisfies the 
photoelectric data as well as the large number of visual minima times accumulated
during the last hundred years.
  Although U Oph is not neglected by photoelectric observers, we wish to 
emphasize that more minima times of secondary eclipse are needed - not only to
improve on light-time orbit, eccentricity and apsidal motion parameters, but
also to ensure that random variations from other causes do not introduce 
systematic errors in their determination. The importance of observing apsidal
motion in eclipsing binaries with reliable absolute dimensions as a test for
theoretical models of stellar structure and stellar evolution theory is well-
known.
  The full details and implications of this period study, including a discussion
of the evolutionary status of U Ophiuchi, will be presented elsewhere
(Kamper, 1985).
   I would like to thank W. Braune, Berlin, for pointing out this system, and
D. Lichtenknecker, Hasselt, for his help in searching the literature.




                        B.-C. KAMPER
                        Berliner Arbeitsgemeinschaft
                        fur Veranderliche Sterne e.V. (BAV)
                        Munsterdamm 90, D-1000 Berlin 41

References:


Frieboes-Conde, H., Herczeg, T.: 1973, Astron. Astrophys. Suppl. Ser. 12, 1. [BIBCODE 1973A&AS...12....1F ]
Herczeg, T.J.: 1980, IAU Symp. No. 88, 89 (Eds. Plavec, Popper and Ulrich). [BIBCODE 1980IAUS...88...89H ]
Huffer, C.M., Kopal, Zd.: 1951, Astrophys. J. 114, 297. [BIBCODE 1951ApJ...114..297H ]
Kamper, B.-C.: 1985, submitted to Astrophysics and Space Science. [BIBCODE 1986Ap&SS.120..167K ]
Koch, R.H., Koegler, C.A.: 1977, Astron. J. 214, 423. [BIBCODE 1977ApJ...214..423K ]
Magalashvili, N.L.: 1949, Abast. Astrofiz. Obs. Byull. 10, 1.
Panchatsaram, T.: 1981, Bull. Astron. Soc. India 9, 139. [BIBCODE 1981BASI....9..139P ]
Parenago, P.P.: 1949, Peremennye Zvezdy 7, 102. [BIBCODE 1949PZ......7..109P ]




  Note: Two phe times of minimum given by Popovici 1971 (IBVS No. 508) must
certainly be attributed to U Peg, not to U Oph.