COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                          Number 3296

                                                    Konkoly Observatory
                                                    Budapest
                                                    28 February 1989

                                                    HU ISSN 0374 - 0676


      A NOTE ON THE PERIOD OF THE W UMa STAR YZ PHOENICIS

The only published data on the star YZ Phe are a light curve and
several timings of minima from Sonneberg by Gessner and Meinunger
(1976). They state that it is a W UMa star and derive an ephemeris
for time of minimum of JD 2436765.622 + 0.3052d E; they note that, due
to the small amplitude of variability, the derived period is
uncertain. This star was put on a programme of observation of W UMa
stars that was commenced at SAAO. The data that were obtained,
however, cover about half a cycle and cannot therefore be analysed in
detail, but some conclusions may be drawn on the period of the star.
   The observations of YZ Phe were made in UBVRIc with the St
Andrews Photometer on the 1.0-m Elizabeth telescope at SAAO, Sutherland. 
These have been transformed on to the standard system through
observations of E-region stars (Menzies et al, 1980). Regular observations 
were also made of comparison and check stars, as listed in
Table 1, which were constant to within 0.01 mag over the period of the
observations. The data, which will be published elsewhere, are shown
in Figure 1. For illustrative purposes, the data have been phased
with a period of 0.2248 d (see below) and the minimum arbitrarily
shifted to phase 0.5. (There appears to be some scatter around
maximum in U; the cause of this is not due to variation in sky
background or transparency and remains unknown).
   The minimum covered by the present observations is at HJD
2445621.39683 +/- 0.00018, using the method of Kwee end van Woerden
(1956).
   The "period" derived from these observations is 0.1124 d; since
this actually corresponds to only half a cycle, then the true period
of YZ Phe is ~0.225 d. This is close to the observed lower limit of
periods for W UMa stars, making YZ Phe one of the shortest period
objects known in its class. Whilst the period of YZ Phe is likely to
have changed, a change as large as thirty per cent is unlikely and
indicates that the value of 0.3052 d was in fact incorrect; more 
probable is that 0.3052 d is a 1 day alias of the true period at that
epoch, 0.2338 d.


                       Table 1

     Colour indices of comparison stars for YZ Phe

                V      B-V     U-B     V-Rc    V-Ic   n   Sp*

HD 10521    8.383    0.360   0.032   0.220   0.442   15   F3V
             .008     .006    .011    .004    .003
HD 10839    9.070    1.156   1.364   0.658   1.232    3   K2III
             .010     .010    .020    .003    .003

* From Houk (1978)


                         [FIGURE 1]

             Figure 1.   Light curves of YZ Phe



                     Table 2

             Colour indices of YZ Phe

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

Max     12.597    0.882     -     0.542   1.028
          .010     .005            .010    .013
Min     13.257    0.910   0.613   0.577   1.093
          .015     .015    .027    .008    .017

   The colours of the comparison and check stars, when transformed
to the Johnson system (Cousins, 1980), are consistent with those of
their spectral types (Johnson, 1966; Houk, 1978), implying that the
reddening is small or negligible. The colours of YZ Phe are listed
in Table 2 (these are the means and s.d. of six measures). These
are not consistent with any one spectral type, the (V-R) and (V-I)
colours implying a slightly later type (K2-K5) than (B-V) and (U-B)
(K0-K2). The (B-V) colour of YZ Phe also appears to be slightly blue
for its period (by ~0.1-0.2 mag), according to the period-colour
relation (Mochnacki, 1985); the short period and of this relation,
however, is poorly defined. The colours nevertheless imply that YZ
Phe is a W-type W UMa star.


                      J H SPENCER JONES

                 S A Astronomical Observatory
                          PO Box 9
                       Observatory 7935
                        South Africa



References

Cousins, A.W.J., 1980. Sth. Afr. astr. Obs. Circ., 1, 234. [BIBCODE 1980SAAOC...1..234C ]
Gessner, H. & Meinunger, I., 1976. Veroff. Stern. Sonneberg 8, 247. [BIBCODE 1976VeSon...8..247G ]
Houk, N., 1978. Michigan Spectral Catalogue Vol. 2. [BIBCODE 1978mcts.book.....H ]
Johnson, H.L., 1966. Ann. Rev. Astr. Astrophys., 4, 193. [BIBCODE 1966ARA&A...4..193J ]
Kwee, K.K. & van Woerden, H., 1956. Bull. astr. Inst. Neth., 12, 327. [BIBCODE 1956BAN....12..327K ]
Menzies, J.W., Banfield, R.M. & Laing, J.D., 1980. Sth. Afr. astr.
    Obs. Circ., 1, 149. [BIBCODE 1980SAAOC...1..149M ]
Mochnacki, S.W., 1985. In Interacting Binaries eds P.P. Eggleton end
    J.E. Pringle p 51. [BIBCODE 1985ib...proc...51M ]