COMMISSION 27 OF THE I. A. U.
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3458 

  	  		  	            Konkoly Observatory
                              	            Budapest
                              	            7 May 1990

                              	            HU ISSN 0374 - 0676



    PHOTOELECTRIC OBSERVATIONS OF XY LEONIS AND ITS PERIOD CHANGES

   XY Leo (BD+18 2307) is a W UMa-type binary (subtype W). Its spectral
type is K0n (Hill et al., 1975). Hrivnak (1985) collected all available,
photoelectric times of minimum light. The O-C diagram has a sine-like shape.
   Hrivnak (1985) admitted among others the possibility that period
changes were abrupt more or less every 10 years with the period remaining
constant between them. Such behaviour was documented for 15 W UMa systems
by Kreiner (1977). Gehlich et al. (1972) first proposed that the variations 
in the O-C values may be due to a third body. This was confirmed by
Barden (1986). He found a BY Draconis-like binary companion to the contact
binary XY Leo. Because the next minimum of the "third body sinusoid" was
predicted in 1990, we carried out observations of this star in B and V 
filters, close to the UBV system. The observations were made using the 0.6 m
telescope equipped with a two-channel photometer (Szymanski and Udalski,
1989) at Mt. Suhora Observatory of Cracow Pedagogical Uníversity, and with
the 0.5 m Cassegrain telescope of the Jagiellonian University Observatory in
Cracow equipped with a one-channel photometer, during the 1989-1990 season.
The differential observations were made with respect to the comparison star
BD+18 2306 and were corrected for the atmospheric extinction using the mean
extinction coefficients for the observatories. The observations allowed us
to determine the times of minima by parabolic fitting to the observational
points, making use of the least-squares method. New times of minimum light
for XY Leo are given in Table I.
   Adding all times of photoelectric minima available in the literature
and a few visual minima observed before JD hel. 2434500 (the ratio of the
weight of the photoelectric minimum to the visual one is equal to 5) and
assuming that the period of XY Leo exhibits sinusoidal changes, a new
ephemeris of minimum times is derived as follows:


Min I JD hel. = 2435484.0222 + 0.28410273 E + 0.0213 sin(0.000243*E + 0.56)
           m.e.         +/-6         +/-2       +/-4       -+/-13     +/-2


Table I. New light minima for XY Leo

JD hel             filter   minimum type    notes

2440000+
7868.6082+/-0.0002   V        I              Cracow
7891.6214+/-0.0002   B        I              Suhora
7896.5933+/-0.0004   V        II             Cracow
7897.5881+/-0.0003   V        I              Cracow
7898.5811+/-0.0002   B        II             Suhora
7899.4337+/-0.0014   B        II             Suhora
7899.5763+/-0.0003   V        I              Suhora
7928.5545+/-0.0004   V        I              Cracow
7928.5551+/-0.0002   V        I              Suhora
7928.6963+/-0.0006   V        II             Suhora
7948.4425+/-0.0003   V        I              Cracow
7968.3300+/-0.0004   V        I              Cracow
7968.4712+/-0.0004   V        II             Cracow
7969.3215+/-0.0008   B        II             Cracow
7969.3226+/-0.0005   V        II             Cracow
7969.4653+/-0.0004   B        I              Cracow
7969.4664+/-0.0004   V        I              Cracow


                              [FIGURE 1]

 Fig. 1. O-C diagram of the times of minimum light for XY Leo


The period indicated by the sinusoidal term is about 20 +/-1 years (what is
equal to 25866 epochs - see Fig. 1.). The ephemeris for the predictíon of
forthcoming minima can be derived from the photoelectric observations made
after JD hel. 2447000, as follows:

          Min I = JD hel 2447612.34748 + 0.2841034 E
                   m.e.         +/- 16       +/- 2

We computed the orbital parameters of the third body, making use of the
gradient-expansion algorithm (Bevington, 1969). Taking into account 
observations before JD hel. 2442000 the results were practically the same as
those obtained by Gehlich, but for observations made after JD hel. 2440000
both the computed period, and the amplitude of theoretical O-C are smaller,
and value of e is near zero, so changes of period of XY Leo are sinusoidal
in quite good approximation.

   This work was supported by a grant from Polish Ministry of Education
(RR I-II).


   JERZY KRZESINSKI ^1, EWA KUCZAWSKA ^2, MARIA KURPINSKA-WINIARSKA^2
   1)                       2)
   Pedagogical University  Astronomical Observatory
   ul. Podchorazych 2      ul. Orla 171
   30-084 Cracow, Poland   30-244 Cracow, Poland


References:

Barden, S.C.: 1986, Ap.J., 317, 333. [BIBCODE 1987ApJ...317..333B ]
Bevington, P.R.: 1969, "Data Reduction and Error Analysis for the Physical
     Sciences", McGraw-Hill Company [BIBCODE 1969drea.book.....B ]
Gehlich, U.K. et al.: 1972, Astron. Astroph. 18, 477. [BIBCODE 1972A&A....18..477G ]
Hill, G. et al.: 1975, Mem.R.A.S., 79, 131. [BIBCODE 1975MmRAS..79..131H ]
Hrivnak, B.J.: 1985, Ap.J. 290, 696 [BIBCODE 1985ApJ...290..696H ]
Kreiner, J.M.: 1977, in IAU Colloquium 42, "The Interaction of Variable
     Stars with Their Environment", ed. R. Kippenhahn et al., p.393 [BIBCODE 1977ivsw.conf..393K ]
Szymanski, M., Udalski, A.: 1989, Acta Astr., 39, 1. [BIBCODE 1989AcA....39....1S ]