COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 3438
               
	               		     Konkoly Observatory
        	                     Budapest
                	             6 March 1990

                        	     HU ISSN 0374 - 0676

       ON THE PERIOD OF THE W UMA STAR YZ PHOENICIS

    From several timings of minima of the W UMa system, YZ
Phoenicis, Gessner & Meinunger (1976) derive a period of
0.3052 d. Spencer Jones (1989) however, has shown that the
period is near 0.225 d, close to the observed lower limit for
periods of W UMa binaries.
    Photoelectric observations of YZ Phe were made in 1989 Sep
- Nov with the 0.5m and 1.0m telescopes of the South African
Astronomical Observatory at Sutherland. Regular observations of
the comparison stars used by Spencer Jones (1989) were made and
all data were transferred to the UBV(RI)c system using observa-
tions of E-region standard stars (Menzies, Cousins, Banfield &
Laing 1989). The mean values for colours and magnitudes of the
comparison stars are given in Table 1 together with standard
deviations of the means. The results for HD 10521 are in very
good agreement with the values given by Spencer Jones (1989) but
for HD 10839, the differences in V, (B-V) end (U-B) are +0.02,
-0.08 and +0.06 (Spencer Jones minus this paper) and it is
possible that HD 10839 is a long-period variable. There is,
however, no evidence in the 1989 date for any variation as big as
0.01 in magnitude or colours end so the YZ Phe data were corrected 
to the mean values of both Table 1 stars.
    A phase-dispersion minimisation technique was applied to
the YZ Phe data and gave a period of 0.234726 d +/- 0.000002 where
the error is estimated from the range of results obtained by
binning the data in different sized phase intervals. The V 
magnitudes from the 1989 data, phased with the above period are
shown in Fig.1. The light curve clearly has unequal maxima and
minima and is typical of a W UMa star. Using the best primary
minimum from the 1989 data and the accurate determination of
primary minimum by Spencer Jones (1989) results in a period
0.23472715 d +/- 0.0000001 where the error corresponds to en error
of 0.001 d in the 9492 cycles between the two primary minima
used. The one cycle aliases (0.23475 d and 0.23470 d) can be
ruled out because adopting these periods results in obvious drift
in the phased magnitudes for the 1989 data.


                       [FIGURE 1]

   Figure 1. V light curve for YZ Phe from 1989 data.


    The times of minima measured in 1989 are listed in Table 2
and a linear least-squares solution, including the Spencer Jones
(1989) minimum For which we adopt n = 0, gives an ephemeris:
       T(primary minimum) = 244 5621.39679 + 0.23472700.n
                                +/-0.00008 +-0.00000008
The observed-calculated (O-C) residuals from this ephemeris are
also listed in Table 2.
    Applying the above period to the Gessner & Meinunger (1976)
timings of minima enables a redetermination of the cycle numbers
for those minima end then a linear least-squares fit to that data
gives a period 0.23472d+/- 0.00003. It does not seem possible
to link the Gessner & Meinunger data to the Table 2 data with
great certainty because of the large separation (~ 37000 cycles)
and because many of the W UMa stars are known to have variable
periods (see, for example, Kreiner 1977) which could cause
aliasing errors. Comparison of the periods derived from the
1989 data and the Gessner & Meinunger data, however, suggest that
the period might be fairly stable on a timescale of years.
   The derived period of 0.234727 d for YZ Phe makes it one
of the shortest period eclipsing binaries known; according to
Mochnacki (1983, Table 2) only CC Com has a shorter period
(0.221 d).


 Table 1. Magnitudes and colours of the comparison stars

                 V    B-V   V-Rc  V-Ic  n  U-B  n

   HD 10521    8.383 0.367 0.219 0.438 29 0.024 15
            +/-0.004 0.003 0.003 0.005    0.005

   HD 10839    9.052 1.235 0.650 1.226 24 1.301 11
               0.004 0.004 0.002 0.003    0.006


 Table 2.     Times of minima of YZ Phe

n              HJD             (O-C )

0        2445621.39683         0.0000 (Spencer Jones 1989)
9250        7792.6220         +0.0004
9253.5      7793.4425         -0.0006
9254        7793.5610         +0.0005
9258.5      7794.6160         -0.0007
9296.5      7803.5355         -0.0009
9313.5      7807.5260         -0.0007
9437        7836.5165         +0.0010
9440.5      7837.3370         -0.0001
9492        7849.4265         +0.0010



                DAVID KILKENNY & FRED MARANG
                South African Astronomical Observatory
                P.O. Box 9, Observatory 7935
                South Africa
                

References:

Gessner, H., Meinunger, I. 1976, Veroff. Stern. Sonneberg 8, 247. [BIBCODE 1976VeSon...8..247G ]
Kreiner, J.M. 1977, I.A.U. Colloquium 42, 393. [BIBCODE 1977ivsw.conf..393K ]
Menzies, J.W., Cousins, A.W.J., Banfield, R.M., Laing, J.D.,
   1989, SAAO Circular 13, 1. [BIBCODE 1989SAAOC..13....1M ]
Mochnacki, S.W., 1983, in Interacting Binaries, p.51 eds. P.P.
   Eggleton & J.E. Pringle, D. Reidel, Dordrecht. [BIBCODE 1985ib...proc...51M ]
Spencer Jones, J.H., 1989, Inf. Bull. Var. Stars No. 3296.