COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 3158

                                      Konkoly Observatory
                                      Budapest
                                      17 March 1988

                                      HU ISSN 0374 - 0676


      W CRU - MINIMUM IN THE GENEVA PHOTOMETRIC SYSTEM


   W Cru = HD 105998 is an enigmatic eclipsing variable. Plavec (1984)
launched the campaign for observation of this star and the reader can find
further information in his paper. Here we publish some of our preliminary
observational results to provide a guidance to spectroscopists about the
most important phases. So far we have no data about the secondary minimum,
but observations continue.

   The observations are performed with the reflecting 70-cm Swiss telescope
at ESO La Silla and the P7 photometer (Burnet 1976, Burnet, Rufener 1979)
for the GENEVA photometric system (Golay 1974 1980, Rufener 1964, 1981).
Figs. 1 to 3 represent part of the light curve close to primary minimum
in colours [V], [U] and [G] respectively. For phase calculation the following
ephemeris was used:
                            
           Min I (HJD) = 2440731.6 + (198.53d).E

    The asymmetry of the light curve with respect to the minimum is obvious.
It would be important to know whether this asymmetry repeats for all eclipses
or if it varies from one eclipse to another. Also the difference in the photometric
behaviour for different colours is quite apparent. The humps on both
sides of the minimum are puzzling. Fig. 4. shows the variation of the
Geneva index [U-B2]. There is lot of symmetry and also asymmetry in
this behaviour. Using this graph theoreticians and spectroscopists may
select the appropriate phase for more detailed study to find out what is
really happening at certain phases. The star is quite bright (8 mag.)
and the period long (198.5 day) and high resolution spectra can be obtained
even with telescopes of moderate size.

    Fig. 5 shows the relationship between magnitude [V] and the index
[U-B2]. Each measured point is represented by two digits showing the
phase multiplied by 100. Thus the number 08 marks (at its center) the
point with phase 0.08. The asymmetry between the descending and ascending
branches is remarkable. Note the differences close to the phases .13 and
.83, and also the loop between phases .17 and .28. If this loop also repeats
at other cycles (epochs) at the same phase this part of the light curve
might be the most interesting one for spectroscopic observations. However,
comparing our light curve with the recent photoelectric observations of
other observers (Marino et al. 1984, Menzies, Jones 1984) we see that the
light curve might be variable and the steps and humps may not always
occur at the same phase. Nevertheless, from our data it also seems that
other indices are changing continuously and smoothly between the phases
.15 and .30, and would deserve further attention of both photometrists
and spectroscopists.


                            [FIGURE 1]

        Fig. 1. Magnitude [V] against phase for W Cru


                            [FIGURE 2]

        Fig. 2. Magnitude [U] against phase for W Cru


                            [FIGURE 3]

        Fig. 3. Magnitude [G] against phase for W Cru


                            [FIGURE 4]

        Fig. 4. Geneva index [U-B2] against phase for W Cru


                            [FIGURE 5]

        Fig. 5. Colour index [U-B2] against magnitude [V], two-digit
                numbers represent the phase multiplied by 100


   Most of the observations were done in 1984/85 by one of the authors
(Z.K.). Since then some observations have been carried out by the constructor
of the photometer, M. Burnet, and occasionally by other observers of the
Geneva Observatory. Further observations are being reduced and they
will be published elsewhere.

                               Z. KVIZ*, F. RUFENER
                               Observatoire de Geneve
                               CH-1290 Sauverny
                               Switzerland



REFERENCES:
Burnet M. 1976, Thesis No. 235, Ecole Polytechnique Federal de Lausanne. [BIBCODE 1976PhDT.......217B ]
Burnet M., Rufener F. 1979, Astron. Astrophys. 74, 54. [BIBCODE 1979A&A....74...54B ]
Golay M., 1974, Introduction to Astronomical Photometry, Reidel, Holland. [BIBCODE 1974ASSL...41.....G ]
        1980, Vistas in Astronomy, 24, 141. [BIBCODE 1980VA.....24..141G ]
Marino et al. 1984, IBVS No. 2582.
Menzies J.W. & Jones J.S. 1984, IBVS No. 2623.
Plavec M. 1984, IBVS No. 2524.
Rufener F. 1964, Publ. Obs. Geneve, Ser. A., 66.
           1981, Astron. Astrophys. Supp. Ser. 45, 207. [BIBCODE 1981A&AS...45..207R ]


* On leave from:
School of Physics, University of New South Wales, P.O. Box 1,
Kensington, NSW, Australia.